Galaxy formation through hierarchical clustering

Astrophysical Journal, 2000

Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterised by over-densities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts -the progenitors of massive cluster ellipticals. By searching for this population at submillimeter (submm) wavelengths we can therefore test both galaxy-and structure-formation models. We have undertaken such a search in the field of a z = 3.8 radio galaxy, 4C 41.17, with the SCUBA submm camera. Our extremely deep 450-and 850-µm maps reveal an order-of-magnitude over-density of luminous submm galaxies compared to typical fields (the likelihood of finding such an over-density in a random field is < 2 × 10 −3 ). The SCUBA galaxies have bolometric luminosities, > 10 13 L ⊙ , which imply star-formation rates (SFRs) consistent with those required to form a massive galaxy in only a few 10 8 years. We also note that this field exhibits an over-density of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the over-densities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a proto-cluster centered on the radio galaxy at z = 3.8, which is also traced by a less-obscured population of Lyman-break galaxies.

Astronomical Journal, 2010

New radial velocity measurements for previously known and newly confirmed globular clusters (GCs) in the nearby massive galaxy NGC 5128 are presented. We have obtained spectroscopy from LDSS-2/Magellan, VIMOS/VLT, and Hydra/CTIO from which we have measured the radial velocities of 218 known, and identified 155 new, GCs. The current sample of confirmed GCs in NGC 5128 is now 605 with 564 of these having radial velocity measurements. We have performed a new kinematic analysis of the GC system that extends out to 45 arcmin in galactocentric radius. We have examined the systemic velocity, projected rotation amplitude and axis, and the projected velocity dispersion of the GCs as functions of galactocentric distance and metallicity. Our results indicate that the metal-poor GCs have a very mild rotation signature of (26 pm 15) km/s. The metal-rich GCs are rotating with a higher, though still small signature of (43 pm 15) km/s around the isophotal major axis of NGC 5128 within 15 arcmin. Their velocity dispersions are consistent within the uncertainties and the profiles appear flat or declining within 20 arcmin. We note the small sample of metal-rich GCs with ages less than 5 Gyr in the literature appear to have different kinematic properties than the old, metal-rich GC subpopulation. The mass and mass-to-light ratios have also been estimated using the GCs as tracer particles for NGC 5128. Out to a distance of 20 arcmin, we have obtained a mass of (5.9 pm 2.0) x 10^(11) M_(sun) and a mass-to-light ratio in the B-band of 16 M_(sun)/L_(B,sun). Combined with previous work on the ages and metallicities of its GCs, as well as properties of its stellar halo, our findings suggest NGC 5128 formed via hierarchical merging over other methods of formation, such as major merging at late times.

Monthly Notices of The Royal Astronomical Society, 2001

Direct and indirect observational evidence leads to the conclusion that high-redshift QSOs did shine in the core of early-type protogalaxies during their main episode of star formation. Exploiting this fact, we derive the rate of formation of this kind of stellar system at high redshift by using the QSO luminosity function. The elemental proportions in elliptical galaxies, the descendants of the QSO hosts, suggest that the star formation was more rapid in more massive objects. We show that this is expected to occur in dark matter haloes, when the processes of cooling and heating are considered. This is also confirmed by comparing the observed submm counts with those derived by coupling the formation rate and the star formation rate of the spheroidal galaxies with a detailed model for their SED evolution. In this scenario SCUBA galaxies and Lyman-break galaxies are early-type protogalaxies forming the bulk of their stars before the onset of QSO activity.

Astronomical Journal, 2011

The outer disk of the Large Magellanic Cloud (LMC) is studied in order to unveil clues about its formation and evolution. Complementing our previous studies in innermost fields (3 kpc lsim R <~ 7 kpc), we obtained deep color-magnitude diagrams in six fields with galactocentric distances from 5.2 kpc to 9.2 kpc and different azimuths. The comparison with isochrones shows that while the oldest population is approximately coeval in all fields, the age of the youngest populations increases with increasing radius. This agrees with the results obtained in the innermost fields. Low-resolution spectroscopy in the infrared Ca II triplet region has been obtained for about 150 stars near the tip of the red giant branch in the same fields. Radial velocities and stellar metallicities have been obtained from these spectra. The metallicity distribution of each field has been analyzed together with those previously studied. The metal content of the most metal-poor objects, which are also the oldest according to the derived age-metallicity relationships, is similar in all fields independently of the galactocentric distance. However, while the metallicity of the most metal-rich objects measured, which are the youngest ones, remains constant in the inner 6 kpc, it decreases with increasing radius from there on. The same is true for the mean metallicity. According to the derived age-metallicity relationships, which are consistent with being the same in all fields, this result may be interpreted as an outside-in formation scheme in opposition with the inside-out scenario predicted by ΛCDM cosmology for a galaxy like the LMC. The analysis of the radial velocities of our sample of giants shows that they follow a rotational cold disk kinematics. The velocity dispersion increases as metallicity decreases indicating that the most metal-poor/oldest objects are distributed in a thicker disk than the most metal-rich/youngest ones in agreement with the findings in other disks such as that of the Milky Way. They do not seem to be part of a hot halo, if one exists in the LMC. Based on observations collected at the European Southern Observatory, Chile, within the observing programs 074.B-0474 and 082.B-0900.

Monthly Notices of The Royal Astronomical Society, 2011

The spectral energy distribution of galaxies is a complex function of the star formation history and geometrical arrangement of stars and gas in galaxies. The computation of the radiative transfer of stellar radiation through the dust distribution is time-consuming. This aspect becomes unacceptable in particular when dealing with the predictions by semi-analytical galaxy formation models populating cosmological volumes, to be then compared with multi-wavelength surveys. Mainly for this aim, we have implemented an artificial neural network algorithm into the spectro-photometric and radiative transfer code GRASIL in order to compute the spectral energy distribution of galaxies in a short computing time. This allows to avoid the adoption of empirical templates that may have nothing to do with the mock galaxies output by models. The ANN has been implemented to compute the dust emission spectrum (the bottleneck of the computation), and separately for the star-forming molecular clouds and the diffuse dust (due to their different properties and dependencies). We have defined the input neurons effectively determining their emission, which means this implementation has a general applicability and is not linked to a particular galaxy formation model. We have trained the net for the disc and spherical geometries, and tested its performance to reproduce the SED of disc and starburst galaxies, as well as for a semi-analytical model for spheroidal galaxies. We have checked that for this model both the SEDs and the galaxy counts in the Herschel bands obtained with the ANN approximation are almost superimposed to the same quantities obtained with the full GRASIL. We conclude that this method appears robust and advantageous, and will present the application to a more complex SAM in another paper.

Astronomical Journal, 2008

We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the homogeneous analysis of 14 deep Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) pointings spanning the range 11.5 kpc lsim R proj lsim 45 kpc. Many of these pointings sample coherent substructure discovered in the course of the Isaac Newton Telescope Wide Field Camera (INT/WFC) imaging survey of M31 while others sample more diffuse structure in the extended disk. We conduct a quantitative comparison of the resolved stellar populations in these fields and identify several striking trends. The color-magnitude diagrams (CMDs), which reach gsim3 mag below the red clump with high completeness, can be classified into two main categories based on their morphologies. "Stream-like" fields, so named for their similarity to the CMD of the giant stellar stream, are characterized by a red clump that slants blueward at fainter magnitudes and an extended horizontal branch. They show no evidence for young populations. On the other hand, "disk-like" fields exhibit rounder red clumps with significant luminosity width, lack an obvious horizontal branch, and show evidence for recent star formation (~0.25-2 Gyr ago). We compare the spatial and line-of-sight distribution of stream-like fields with a recent simulation of the giant stream progenitor orbit and find an excellent agreement. These fields are found across much of the inner halo of M31, and attest to the high degree of pollution caused by this event. Disk-like material resides in the extended disk structure of M31 and is detected here up to R proj ~ 44 kpc; the uniform populations in these fields, including the ubiquitous presence of young populations, and the strong rotation reported elsewhere are most consistent with a scenario in which this structure has formed through heating and disruption of the existing thin disk, perhaps due to the impact of the giant stream progenitor. Our comparative analysis sheds new light on the likely composition of two of the ultra-deep pointings formerly presented as pure outer disk and pure halo in the literature. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Astrophysical Journal, 2003

We use the weak gravitational lensing effect to study the mass distribution and dynamical state of a sample of 24 X-ray luminous clusters of galaxies ($0.05<z<0.31$) observed with the VLT-FORS1 under homogeneous sky conditions and subarsecond image quality. The galaxy shapes were measured after deconvolution with a locally determined point-spread-function, while the two-dimensional mass distributions of the clusters were computed using an algorithm based on the maximum entropy method. By comparing the mass and light distributions of the clusters in our sample, we find that their mass centers, for the majority of the clusters, is consistent with the positions of optical centers. We find that some clusters present significant mass substructures which generally have optical counterparts. At least in one cluster (Abell 1451), we detect a mass substructure without an obvious luminous counterpart. The radial profile of the shear of the clusters was fitted using circular and elliptical isothermal elliptical distributions, which allowed the finding of a strong correlation between the orientation of the major-axis of the matter distribution and the corresponding major-axes of the brightest cluster galaxy light-profiles. Estimates of how close to dynamical relaxation are these clusters were obtained through comparison of our weak-lensing mass measurements with the x-ray and velocity dispersion determinations available in the literature. We find that clusters with intra-cluster gas colder than 8 keV show a good agreement between the different mass determinations, but clusters with gas hotter than 8 keV present discrepant mass values.

Astrophysical Journal, 2003

We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51 arcmin (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at V = 30.7 mag. The CMD, constructed from approximately 300,000 stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as chi-squared fitting to a finely-spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (approximately 30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population, as well as a significant globular-cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo.

Astrophysical Journal, 2004

Hubble Space Telescope images of high-redshift galaxies selected via color and photometric redshifts are used to examine the size and axial-ratio distribution of galaxies as a function of redshift at lookback times t > 8 Gyr. These parameters are measured at rest-frame UV wavelengths (1200-2000 Angstroms) on images with a rest-frame resolution of less than 0.8 kpc. Galaxy radii are found to scale with redshift approximately as the inverse of the Hubble parameter H(z). This is in accord with the theoretical expectation that the typical sizes of the luminous parts of galaxies should track the expected evolution in the virial radius of dark-matter halos. The mean ratio of semi-major to semi-minor axis for a bright well-resolved sample of galaxies at z ~ 4 is b/a = 0.65, suggesting that these Lyman break galaxies are not drawn from a spheroidal population. However the median concentration index of this sample is C = 3.5, which is closer to the typical concentration indices, C ~ 4, of nearby elliptical galaxies than to the values, C<2 for local disk galaxies of type Sb and later.

Astrophysical Journal, 2006

We present the evolution of the luminosity-size and stellar mass-size relations of luminous (L_V>3.4x10^10h_70^-2L_sun) and of massive (M_*>3x10^10h_70^-2M_sun) galaxies in the last ~11 Gyr. We use very deep near-infrared images of the Hubble Deep Field-South and the MS1054-03 field in the J_s, H and K_s bands from FIRES to retrieve the sizes in the optical rest-frame for galaxies with z>1. We combine our results with those from GEMS at 0.2<z<1 and SDSS at z~0.1 to achieve a comprehensive picture of the optical rest-frame size evolution from z=0 to z=3. Galaxies are differentiated according to their light concentration using the Sersic index n. For less concentrated objects, the galaxies at a given luminosity were typically ~3+-0.5 (+-2 sigma) times smaller at z~2.5 than those we see today. The stellar mass-size relation has evolved less: the mean size at a given stellar mass was \~2+-0.5 times smaller at z~2.5, evolving proportional to (1+z)^{-0.40+-0.06}. Simple scaling relations between dark matter halos and baryons in a hierarchical cosmogony predict a stronger (although consistent within the error bars) than observed evolution of the stellar mass-size relation. The observed luminosity-size evolution out to z~2.5 matches well recent infall model predictions for Milky-Way type objects. For low-n galaxies, the evolution of the stellar mass-size relation would follow naturally if the individual galaxies grow inside-out. For highly concentrated objects, the situation is as follows: at a given luminosity, these galaxies were ~2.7+-1.1 times smaller at z~2.5 (or put differently, were typically ~2.2+-0.7 mag brighter at a given size than they are today), and at a given stellar mass the size has evolved proportional to (1+z)^{-0.45+-0.10}.

Astronomy and Astrophysics, 2002

The cosmic star formation history in Cold Dark Matter dominated cosmological scenarios is studied by means of hydrodynamical numerical simulations. In particular, we explore a low density model with a Λ-term and two high density models with different power spectra, all of them being spatially flat. Our simulations employ a fully nonlinear N-body and Eulerian hydrodynamics algorithm with a model for star formation and supernovae feedback that depends on two phenomenological parameters determined in agreement with previous papers. We find a nearly constant star formation rate beyond z = 1, and we discuss which facts may determine the decrease in the SFR from z = 1 to the present epoch. The Λ-term cosmology with realistic parameters for star formation and feedback best reproduces the observed star formation history.

Astronomy & Astrophysics, 2011

We present a toy model of the growth of galaxies within dark matter haloes via gas accretion and mergers. We begin with sub-halo merger trees from a very high-resolution dark matter cosmological simulation. Three physical processes cause the evolution of the baryons to differ from that of the dark matter by suppressing gas accretion and star formation: gravitational shock heating coupled to black hole feedback above a critical halo mass of M shock ∼ 10 12 M ⊙ ; the reionisation of the intergalactic medium in haloes with circular velocities below v reion ∼ 40 km s −1 ; stellar feedback in haloes below v SN ∼ 120 km s −1 .

Monthly Notices of the Royal Astronomical Society, 2012

Using a statistical sample of dark matter haloes drawn from a suite of cosmological Nbody simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion and merging history on two commonly used measures of its dynamical state, the virial ratio η and the centre of mass offset ∆r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift z form correlates with its virial mass M vir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing M vir and decreasing z form . We then show that both η and ∆r increase with increasing M vir and decreasing z form , which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both η and ∆r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that ∆r provides a more robust and better defined measure of dynamical state for use in cosmological N -body simulations at z ≃ 0. We find that ∆r 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.

Monthly Notices of the Royal Astronomical Society, 2013

We present a study of a comparison of spin distributions of subhaloes found associated with a host halo. The subhaloes are found within two cosmological simulation families of Milky Way-like galaxies, namely the Aquarius and GHALO simulations. These two simulations use different gravity codes and cosmologies. We employ ten different substructure finders, which span a wide range of methodologies from simple overdensity in configuration space to full 6-d phase space analysis of particles. We subject the results to a common post-processing pipeline to analyse the results in a consistent manner, recovering the dimensionless spin parameter. We find that spin distribution is an excellent indicator of how well the removal of background particles (unbinding) has been carried out. We also find that the spin distribution decreases for substructure the nearer they are to the host halo's, and that the value of the spin parameter rises with enclosed mass towards the edge of the substructure. Finally subhaloes are less rotationally supported than field haloes, with the peak of the spin distribution having a lower spin parameter.

Monthly Notices of the Royal Astronomical Society, 2010

The spin probability distribution of Dark Matter haloes has often been modelled as being very near to a lognormal. Most of the theoretical attempts to explain its origin and evolution invoke some hypotheses concerning the influence of tidal interactions or merging on haloes. Here we apply a very general statistical theorem introduced by Cramér (1936) to study the origin of the deviations from the reference lognormal shape: we find that these deviations originate from correlations between two quantities entering the definition of spin, namely the ratio J/M 5/2 (which depends only on mass) and the modulus E of the total (gravitational + kinetic) energy. To reach this conclusion, we have made usage of the results deduced from two high spatial-and mass resolution simulations. Our simulations cover a relatively small volume and produce a sample of more than 16,000 gravitationally bound haloes, each traced by at least 300 particles. We verify that our results are stable to different systematics, by comparing our results with those derived by the GIF2 and by a more recent simulation performed by Macciò et al. We find that the spin probability distribution function shows systematic deviations from a lognormal, at all redshifts z 1. These deviations depend on mass and redshift: at small masses they change little with redshift, and also the best lognormal fits are more stable. The J − M relationship is well described by a power law of exponent α very near to the linear theory prediction (α = 5/3), but systematically lower than this at z 0.3. We argue that the fact that deviations from a lognormal PDF are present only for high-spin haloes could point to a role of large-scale tidal fields in the evolution of the spin PDF.

Monthly Notices of the Royal Astronomical Society, 2013

We use the Millennium Simulation database to compare how different versions of the Durham and Munich semi-analytical galaxy formation models populate dark matter haloes with galaxies. The models follow the same physical processes but differ in how these are implemented. All of the models we consider use the Millennium N-body Simulation; however, the Durham and Munich groups use independent algorithms to construct halo merger histories from the simulation output. We compare the predicted halo occupation distributions (HODs) and correlation functions for galaxy samples defined by stellar mass, cold gas mass and star formation rate. The model predictions for the HOD are remarkably similar for samples ranked by stellar mass. The predicted bias averaged over pair separations in the range 5 − 25h −1 Mpc is consistent between models to within 10%. At small pair separations there is a clear difference in the predicted clustering. This arises because the Durham models allow some satellite galaxies to merge with the central galaxy in a halo when they are still associated with resolved dark matter subhaloes. The agreement between the models is less good for samples defined by cold gas mass or star formation rate, with the spread in predicted galaxy bias reaching 20% and the small scale clustering differing by an order of magnitude, reflecting the uncertainty in the modelling of star formation. The model predictions in these cases are nevertheless qualitatively similar, with a markedly shallower slope for the correlation function than is found for stellar mass selected samples and with the HOD displaying an asymmetric peak for central galaxies. We provide illustrative parametric fits to the HODs predicted by the models. Our results reveal the current limitations on how well we can predict galaxy bias in a fixed cosmology, which has implications for the interpretation of constraints on the physics of galaxy formation from galaxy clustering measurements and the ability of future galaxy surveys to measure dark energy.

Monthly Notices of the Royal Astronomical Society, 2001

We present a new model for the X-ray properties of the intracluster medium that explicitly includes heating of the gas by the energy released during the evolution of cluster galaxies. We calculate the evolution of clusters by combining the semi-analytic model of galaxy formation of Cole et al. with a simple model for the radial profile of the intracluster gas. We focus on the cluster X-ray luminosity function and on the relation between X-ray temperature and luminosity (the T-L relation). Observations of these properties are known to disagree with predictions based on scaling relations which neglect gas cooling and heating processes. We show that cooling alone is not enough to account for the flatness of the observed T-L relation or for the lack of strong redshift evolution in the observed X-ray luminosity function. Gas heating, on the other hand, can solve these two problems: in the ΛCDM cosmology, our model reproduces fairly well the T-L relation and the X-ray luminosity function. Furthermore, it predicts only weak evolution in these two properties out to z = 0.5, in agreement with recent observational data. A successful model requires an energy input of 1-2 ×10 49 ergs per solar mass of stars formed. This is comparable to the total energy released by the supernovae associated with the formation of the cluster galaxies. Thus, unless the transfer of supernovae energy to the intracluster gas is very (perhaps unrealistically) efficient, additional sources of energy, such as mechanical energy from AGN winds are required. However, the amplification of an initial energy input by the response of the intracluster medium to protocluster mergers might ease the energy requirements. Our model makes definite predictions for the X-ray properties of groups and clusters at high redshift. Some of these, such as the T-L relation at z ≃ 1, may soon be tested with data from the Chandra and Newton satellites.

Monthly Notices of the Royal Astronomical Society, 2001

We compare the statistical properties of galaxies found in two different models of hierarchical galaxy formation: the semi-analytic model of Cole et al. and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al. These two techniques model gas processes very differently: by approximate, analytic methods in the case of the semi-analytic model, and by direct numerical integration of the equations of hydrodynamics for a set of discrete particles in the case of SPH. Using a 'stripped-down' version of the semi-analytic model which mimics the resolution of the SPH simulations and excludes physical processes not included in them, we find that the two models produce an ensemble of galaxies with remarkably similar properties, although there are some differences in the gas cooling rates and in the number of galaxies that populate halos of different mass. The full semi-analytic model, which has effectively no resolution limit and includes a treatment of star formation and supernovae feedback, produces somewhat different (but readily understandable) results. Our comparison demonstrates that, on the whole, SPH simulations and semi-analytic models give similar results for the thermodynamic evolution of cooling gas in cosmological volumes. Agreement is particularly good for the present-day global fractions of hot gas, cold dense (i.e. galactic) gas and uncollapsed gas, for which the SPH and strippeddown semi-analytic calculations differ by at most 25%. In the most massive halos, the stripped-down semi-analytic model predicts, on the whole, up to 50% less gas in galaxies than is seen in the SPH simulations. The two techniques apportion this cold gas somewhat differently amongst galaxies in a given halo. This difference can be tracked down to the greater cooling rate in massive halos in the SPH simulation compared to the semi-analytic model. The galaxy correlation functions in the two models agree to better than about 50% over most pair separations and evolve with redshift in very similar ways. Our comparison demonstrates that these different techniques for modelling galaxy formation produce results that are broadly consistent with each other and highlights areas where further study is required.

Monthly Notices of the Royal Astronomical Society, 1999

Measurements of galaxy clustering are now becoming possible over a range of redshifts out to z ∼ 3. We use a semi-analytic model of galaxy formation to compute the expected evolution of the galaxy correlation function with redshift. We illustrate how the degree of clustering evolution is sensitive to the details of sample selection. For a fixed apparent magnitude limit, galaxies selected at higher redshifts are located in progressively rarer dark matter haloes, compared with the general population of galaxies in place at each redshift. As a result these galaxies are highly biased tracers of the underlying dark matter distribution and exhibit stronger clustering than the dark matter. In general, the correlation length measured in comoving units, decreases at first with increasing redshift, before increasing again at higher redshift. We show that the ǫ-model often used to interpret the angular correlation function of faint galaxies gives an inadequate description of the evolution of clustering, and offers no physical insight into the clustering process. We compare our predictions with those of a simple, popular model in which a one-to-one correspondence between galaxies and dark halos is assumed. Qualitatively, this model reproduces the correct evolutionary behaviour at high redshift, but the quantitative results can be significantly in error. Our theoretical expectations are in good agreement with the high redshift clustering data of Carlberg et al. and Postman et al. but are higher than the measurements of Le Fèvre et al.

Monthly Notices of the Royal Astronomical Society, 2011

The distribution of cold gas in dark matter haloes is driven by key processes in galaxy formation: gas cooling, galaxy mergers, star formation and reheating of gas by supernovae. We compare the predictions of four different galaxy formation models for the spatial distribution of cold gas. We find that satellite galaxies make little contribution to the abundance or clustering strength of cold gas selected samples, and are far less important than they are in optically selected samples. The halo occupation distribution function of present-day central galaxies with cold gas mass > 10 9 h −1 M ⊙ is peaked around a halo mass of ≈ 10 11 h −1 M ⊙ , a scale that is set by the AGN suppression of gas cooling. The model predictions for the projected correlation function are in good agreement with measurements from the HI Parkes All-Sky Survey. We compare the effective volume of possible surveys with the Square Kilometre Array † with those expected for a redshift survey in the near-infrared. Future redshift surveys using neutral hydrogen emission will be competitive with the most ambitious spectroscopic surveys planned in the near-infrared.

Monthly Notices of the Royal Astronomical Society, 2009

We describe a method for constructing mock galaxy catalogues which are well suited for use in conjunction with large photometric surveys. We use the semi-analytic galaxy formation model of Bower et al. implemented in the Millennium N-body simulation of the evolution of dark matter clustering in a ΛCDM cosmology. We apply our method to the specific case of the surveys soon to commence with PS1, the first of 4 telescopes planned for the Pan-STARRS system. PS1 has 5 photometric bands, g, r, i, z and y and will carry out an all-sky "3π" survey and a medium deep survey (MDS) over 84 sq. deg. We calculate the expected magnitude limits for extended sources in the two surveys. We find that, after 3 years, the 3π survey will have detected over 10 8 galaxies in all 5 bands, 10 million of which will lie at redshift z > 0.9, while the MDS will have detected over 10 7 galaxies with 0.5 million lying at z > 2. These numbers at least double if detection in the shallowest band, y is not required. We then evaluate the accuracy of photometric redshifts estimated using an off-the-shelf photo-z code. With the grizy bands alone it is possible to achieve an accuracy in the 3π survey of ∆z/(1 + z) ∼ 0.06 in the range 0.25 < z < 0.8, which could be reduced by about 15% using near infrared photometry from the UKIDDS survey, but would increase by about 25% for the deeper sample without the y band photometry. For the MDS an accuracy of ∆z/(1 + z) ∼ 0.05 is achievable for 0.02 < z < 1.5 using grizy. A dramatic improvement in accuracy is possible by selecting only red galaxies. In this case, ∆z/(1 + z) ∼ 0.02 − 0.04 is achievable for ∼100 million galaxies at 0.4 < z < 1.1 in the 3π survey and for 30 million galaxies in the MDS at 0.4 < z < 2. We investigate the effect of using photometric redshifts in the estimate of the baryonic acoustic oscillation scale. We find that PS1 will achieve a similar accuracy in this estimate as a spectroscopic survey of 20 million galaxies.

Monthly Notices of the Royal Astronomical Society, 2009

The clustering amplitude of galaxies depends on their intrinsic luminosity. We compare the properties of publicly available galaxy formation models with clustering measurements from the two-degree field galaxy redshift survey. The model predictions show the same qualitative behaviour as the data but fail to match the observations at the level of accuracy at which current measurements can be made. We demonstrate that this is due to the model producing too many satellite galaxies in massive haloes. We implement simple models to describe two new processes, satellite-satellite mergers and the tidal dissolution of satellites to investigate their impact on the predicted clustering. We find that both processes need to be included in order to produce a model which matches the observations.

Monthly Notices of the Royal Astronomical Society: Letters, 2005

We investigate the metal enrichment of elliptical galaxies in the framework of hierarchical models of galaxy formation. The semi-analytical model we use in this Letter, which has been used to study the metal enrichment of the intracluster medium (ICM) by Nagashima et al., includes the effects of flows of gas and metals both into and out of galaxies and the processes of metal enrichment due to both type Ia and type II supernovae. We adopt a solar neighbourhood IMF for star formation in discs, but consider models in which starbursts have either a solar neighbourhood IMF or a top-heavy IMF. We find that the α-element abundance in ellipticals is consistent with observed values only if the top-heavy IMF is used. This result is consistent with our previous study on the metal enrichment of the ICM. We also discuss the abundance ratio of α elements to iron as a function of velocity dispersion and metallicity. We find that models with a top-heavy IMF match the α/Fe ratios observed in typical L * ellipticals, but none of the models reproduce the observed increase of α/Fe with velocity dispersion.

Publications of the Astronomical Society of Australia, 2013

Large surveys of the local Universe have shown that galaxies with different intrinsic properties, such as colour, luminosity and morphological type display a range of clustering amplitudes. Galaxies are therefore not faithful tracers of the underlying matter distribution. This modulation of galaxy clustering, called bias, contains information about the physics behind galaxy formation. It is also a systematic to be overcome before the large-scale structure of the Universe can be used as a cosmological probe. Two types of approaches have been developed to model the clustering of galaxies. The first class is empirical and filters or weights the distribution of dark matter to reproduce the measured clustering. In the second approach an attempt is made to model the physics which governs fate of baryons in order to predict the number of galaxies in dark matter haloes. I will review the development of both approaches and summarize what we have learnt about galaxy bias.

The Astrophysical Journal, 2003

We investigate the physical mechanisms that shape the luminosity function of galaxies in hierarchical clustering models. Beginning with the mass function of dark matter halos in the ΛCDM cosmology, we show, in incremental steps, how gas cooling, photoionization at high redshift, feedback processes, galaxy merging and thermal conduction affect the shape of the luminosity function. We consider three processes whereby supernovae and stellar wind energy can affect the forming galaxy: (1) the reheating of cold disk gas to the halo temperature; (2) expansion of the hot, diffuse halo gas; (3) complete expulsion of cold disk gas from the halo. We demonstrate that while feedback of form (1) is able to flatten the faint end of the galaxy luminosity function, this process alone does not produce the sharp cut-off observed at large luminosities. Feedback of form is also unable to solve the problem at the bright end of the luminosity function. The relative paucity of very bright galaxies can only be explained if cooling in massive halos is strongly suppressed. This might happen if thermal conduction near the centres of halos was very efficient, or if a substantial amount of gas was expelled from halos by process (3) above. Conduction is a promising mechanism, but an uncomfortably high efficiency is required to suppress cooling to the desired level. If, instead, superwinds are responsible for the lack of bright galaxies, then the total energy budget required to obtain a good match to the galaxy luminosity function greatly exceeds the energy available from supernova explosions. The mechanism is only viable if the formation of central supermassive black holes and associated energy generation play a crucial role in limiting the amount of stars that form in the host galaxy. The models that best reproduce the galaxy luminosity function, also give reasonable approximations to the Tully-Fisher relation and the galaxy autocorrelation function.

The Astrophysical Journal, 1998

We use a semi-analytic model of galaxy formation in hierarchical clustering theories to interpret recent data on galaxy formation and evolution, focussing primarily on the recently discovered population of Lyman-break galaxies at z ≃ 3. For a variety of cold dark matter (CDM) cosmologies we construct mock galaxy catalogues subject to identical selection criteria to those applied to the real data. We find that the expected number of Lyman-break galaxies is very sensitive to the assumed stellar initial mass function and to the normalization of the primordial power spectrum. For reasonable choices of these and other model parameters, it is possible to reproduce the observed abundance of Lyman-break galaxies in CDM models with Ω 0 = 1 and with Ω 0 < 1. The characteristic masses, circular velocities and starformation rates of the model Lyman-break galaxies depend somewhat on the values of the cosmological parameters but are broadly in agreement with available data. These galaxies generally form from rare peaks at high redshift and, as a result, their spatial distribution is strongly biased, with a typical bias parameter, b ≃ 4, and a comoving correlation length, r 0 ≃ 4h −1 Mpc. The typical sizes of these galaxies, ∼ 0.5h −1 kpc, are substantially smaller than those of present day bright galaxies. In combination with data at lower redshifts, the Lyman-break galaxies can be used to trace the cosmic star formation history. We compare theoretical predictions for this history with a compilation of recent data. The observational data match the theoretical predictions reasonably well, both for the distribution of star formation rates at various redshifts and for the integrated star formation rate as a function of redshift. Most galaxies (in our models and in the data) never experience star formation rates in excess of a few solar masses per year. Our models predict that even at z = 5, the integrated star formation rate is similar to that measured locally, although less than 1% of all the stars have formed prior to this redshift. The weak dependence of the predicted star formation histories on cosmological parameters allows us to propose a fairly general interpretation of the significance of the Lyman-break galaxies as the first galaxy-sized objects that experience significant amounts of star formation. These galaxies mark the onset of the epoch of galaxy formation that continues into the present day. The basic ingredients of a consistent picture of galaxy formation may well be now in place.

The Astrophysical Journal, 2003

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P (N |M ) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative spatial and velocity distributions of galaxies and dark matter within halos. We calculate and compare the HODs predicted by a smoothed particle hydrodynamics (SPH) simulation of a ΛCDM cosmological model (cold dark matter with a cosmological constant) and by a semi-analytic galaxy formation model applied to the same cosmology. Although the two methods predict different galaxy mass functions, their HOD predictions for samples of the same space density agree remarkably well. In a sample defined by a baryonic mass threshold, the mean occupation function N M exhibits a sharp cutoff at low halo masses, a slowly rising plateau in which N climbs from one to two over nearly a decade in halo mass, and a more steeply rising, high occupancy regime at high halo mass. In the low occupancy regime, the factorial moments N (N − 1) and N (N − 1)(N − 2) are well below the values N 2 and N 3 expected for Poisson statistics, with important consequences for the small scale behavior of the 2-and 3-point correlation functions. The HOD depends strongly on galaxy age, with high mass halos populated mainly by old galaxies and low mass halos by young galaxies. The distribution of galaxies within SPH halos supports the assumptions usually made in semi-analytic calculations: the most massive galaxy lies close to the halo center and moves near the halo's mean velocity, while the remaining, satellite galaxies have the same radial profile and velocity dispersion as the dark matter. The mean occupation at fixed halo mass in the SPH simulation is independent of the halo's larger scale environment, supporting both the merger tree approach of the semi-analytic method and the claim that the HOD provides

The Astrophysical Journal, 2005

The halo occupation distribution (HOD) describes the relation between galaxies and dark matter at the level of individual dark matter halos. The properties of galaxies residing at the centers of halos differ from those of satellite galaxies because of differences in their formation histories. Using a smoothed particle hydrodynamics (SPH) simulation and a semi-analytic (SA) galaxy formation model, we examine the separate contributions of central and satellite galaxies to the HOD, more specifically to the probability P (N |M ) that a halo of virial mass M contains N galaxies of a particular class. In agreement with earlier results for dark matter subhalos, we find that the mean occupation function N M for galaxies above a baryonic mass threshold can be approximated by a step function for central galaxies plus a power law for satellites, and that the distribution of satellite numbers is close to Poisson at fixed halo mass. Since the number of central galaxies is always zero or one, the width of P (N |M ) is narrower than a Poisson distribution at low N and approaches Poisson at high N . For galaxy samples defined by different baryonic mass thresholds, there is a nearly linear relation between the minimum halo mass M min required to host a central galaxy and the mass M 1 at which an average halo hosts one satellite, with M 1 ≈ 14M min (SPH) or M 1 ≈ 18M min (SA). The stellar population age of central galaxies correlates with halo mass, and this -2correlation explains much of the age-dependence of the galaxy HOD. The mean occupation number of young galaxies exhibits a local minimum at M ∼ 10M min where halos are too massive to host a young central galaxy but not massive enough to host satellites. Using the SA model, we show that the conditional galaxy mass function at fixed halo mass cannot be described by a Schechter function because central galaxies produce a "bump" at high masses. We suggest parameterizations for the HOD and the conditional luminosity function that can be used to model observed galaxy clustering. Many of our predictions are in good agreement with recent results inferred from clustering in the Sloan Digital Sky Survey.

The Astrophysical Journal, 2000

We present a new evolutionary model for predicting the far-UV to sub-mm properties of the galaxy population. This combines a semi-analytic galaxy formation model based on hierarchical clustering (GALFORM, Cole et al. 2000) with a spectro-photometric code which includes dust reprocessing (GRASIL, Silva et al. 1998). The former provides the star formation and metal enrichment histories, together with the gas mass and various geometrical parameters, for a representative sample of galaxies formed in different density environments. These quantities, together with a few other assumptions concerning the spatial distribution of dust and its optical properties, allow us to model the spectral energy distributions (SEDs) of galaxies, taking into account stellar emission and also dust extinction (absorption plus scattering) and re-emission. In the spectro-photometric code dust is considered only in the disk, but the general radiation field is contributed by both the disk and the bulge components with their own distinct age and metallicity distributions. Two phases are considered for the dust: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. The model includes both galaxies forming stars quiescently in disks, and starbursts triggered by galaxy mergers. We test our models against the observed spectro-photometric properties of galaxies in the local Universe, assuming a CDM cosmology with Ω 0 = 0.3 and Λ 0 = 0.7. The models reproduce fairly well the SEDs of normal spirals and starbursts from the far-UV to the sub-mm, and their internal extinction properties. The starbursts follow the observed relationship between the FIR to UV luminosity ratio and the slope of the UV continuum. They also reproduce the observed starburst attenuation law ). This result is remarkable, because we use a dust mixture which reproduces the Milky Way extinction law. It suggests that the observed attenuation law is closely related to the geometry of the stars and dust. We compute galaxy luminosity functions over a wide range of wavelengths, which turn out to be in good agreement with observational data in the UV (2000Å), in the B and K bands, and in the IR (12 − 100µm). Finally, we investigate the reliability of some star formation indicators which are based on the properties of the continuum SEDs of galaxies. The UV continuum turns out to be a poor star formation indicator for our models, whilst the infrared luminosity is much more reliable.

Nature, 1998

The discovery[1] of a population of young galaxies at an epoch when the universe was about one tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models[2] this is the population of primeval galaxies that built themselves up to the size of present-day galaxies through the process of repeated mergers. But the recent detection [3] of a large concentration of these primeval galaxies appears to be incompatible with hierarchical clustering models, which generally predict that clusters of this size are fully formed later in time. Here we use a combination of two powerful theoretical techniques -semi-analytic modelling and N-body simulations-to show that such large concentrations should be quite common in a universe dominated by cold dark matter, and that they are the progenitors of the rich galaxy clusters seen today. We predict the clustering properties of primeval galaxies which should, when compared with data that will be collected in the near future, test our understanding of galaxy formation within the framework of a universe dominated by cold dark matter.

The Astronomy and Astrophysics Review, 2014

Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship between stellar mass, metallicity, and star formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.

The Astrophysical Journal, 2006

We present a joint gravitational lensing and stellar dynamical analysis of fifteen massive field earlytype galaxies -selected from the Sloan Lens ACS (SLACS) Survey -using Hubble Space Telescope ACS images and luminosity weighted stellar velocity dispersions obtained from the Sloan Digital Sky Survey database. The sample of lens galaxies is well-defined (see Paper I), with a redshift range of z=0.06-0.33 and an average stellar velocity dispersion of σ ap = 263 km s −1 (rms of 44 km s −1 ) inside a 3-arcsec fiber diameter. The following numerical results are found: (i) A joint-likelihood gives an average logarithmic density slope for the total mass density of γ ′ = 2.01 +0.02 −0.03 (68% C.L.; ρ tot ∝ r −γ ′ ) inside R Einst = 4.2 ± 0.4 kpc (rms of 1.6 kpc). The inferred intrinsic rms spread in logarithmic density slopes is σ γ ′ = 0.12, which might still include some minor systematic uncertainties. A range for the stellar anisotropy parameter

The Astrophysical Journal, 2004

We present a detailed comparison of structural properties in the rest-frame V -band of cluster and field galaxies, selected and analyzed in the same manner, to test the hypothesis that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 gigayears) transformation of infalling, field spirals into red, cluster early-types. Specifically, we have selected ∼ 140 galaxies from three nearby Abell clusters (A85, A496 and A754) that have colors significantly bluer than the red sequence population, and compared them to ∼ 80 field galaxies with similar colors and luminosities from Jansen et al. (2000, ApJS, 126, 271). The comparison is based on the hypothesis that recent (1-4 gigayears) cluster arrivals were originally blue and star-forming, then stopped forming stars to dim and redden in a few gigayears. For the comparison we quantify galaxy internal structure and morphology from two-dimensional bulge/disk decompositions using GIM2D.

Monthly Notices of the Royal Astronomical Society, 2002

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of ERO J003707+0909.5, the brightest of three gravitationally-lensed images of an Extremely Red Object (ERO) at z = 1.6, in the field of the massive cluster A 68 (z = 0.255). We exploit the superlative resolution of our HST data and the enhanced spatial resolution and sensitivity afforded by the lens amplification to reconstruct the source-plane properties of this ERO. Our morphological and photometric analysis reveals that ERO J003707 is an L ⋆ early-type disk-galaxy and we estimate that ∼ 10 per cent of EROs with (R − K) ≥ 5.3 and K ≤ 21 may have similar properties. The unique association of passive EROs with elliptical galaxies therefore appears to be too simplistic. We speculate on the evolution of ERO J003707: if gas continues to cool onto this galaxy in the manner predicted by hierarchical galaxy formation models, then by the present day, ERO J003707 could evolve into a very luminous spiral galaxy.

Monthly Notices of the Royal Astronomical Society, 2004

The 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue of ∼ 29000 objects is used to study the luminous content of galaxy systems of various sizes. Mock galaxy catalogues constructed from cosmological simulations are used to gauge the accuracy with which intrinsic group properties can be recovered. It is found that a Schechter function is a reasonable fit to the galaxy luminosity functions in groups of different mass in the real data, and that the characteristic luminosity L * is larger for more massive groups. However, the mock data show that the shape of the recovered luminosity function is expected to differ from the true shape, and this must be allowed for when interpreting the data. Luminosity function results are presented in both the b J and r F wavebands. The variation of halo mass-to-light ratio, Υ, with group size is studied in both these wavebands. A robust trend of increasing Υ with increasing group luminosity is found in the 2PIGG data. Going from groups with b J luminosities equal to 10 10 h −2 L ⊙ to those 100 times more luminous, the typical b J -band mass-to-light ratio increases by a factor of 5, whereas the r F -band mass-to-light ratio grows by a factor of 3.5. These trends agree well with the predictions of the simulations which also predict a minimum in the mass-to-light ratio on a scale roughly corresponding to the Local Group. Our data indicate that if such a minimum exists, then it must occur at L ∼ < 10 10 h −2 L ⊙ , below the range accurately probed by the 2PIGG catalogue. According to the mock data, the b J mass-to-light ratios of the largest groups are expected to be approximately 1.1 times the global value. Assuming that this correction applies to the real data, the mean b J luminosity density of the Universe yields an estimate of Ω m = 0.26 ± 0.03. Various possible sources of systematic error are considered, with the conclusion that these can affect the estimate of Ω m by up to 20%.

The Astrophysical Journal, 2007

We present a quantitative analysis of the morphologies for 199 nearby galaxies as parameterized with measurements of the concentration, asymmetry, and clumpiness (CAS) parameters at wavelengths from 0.15-0.85µm. We find that these CAS parameters depend on both galaxy type and the wavelength of observation. As such, we use them to obtain a quantitative measure of the "morphological kcorrection", i.e., the change in appearance of a galaxy with rest-frame wavelength. Whereas early-type galaxies (E-S0) appear about the same at all wavelengths longward of the Balmer break, there is a mild but significantly-determined wavelength-dependence of the CAS parameters for galaxies types later than S0, which generally become less concentrated, and more asymmetric and clumpy toward shorter wavelengths. Also, as a merger progresses from pre-merger via major-merger to mergerremnant stages, it evolves through the CAS parameter space, becoming first less concentrated and more asymmetric and clumpy, and then returning towards the "locus" of normal galaxies. The final merger products are, on average, much more concentrated than normal spiral galaxies.

The Astrophysical Journal Supplement Series, 2007

We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Herein, we compare GALEX UV and visible-NIR images of 189 nearby (D<40 Mpc) S0-Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalogue of XUV-disk galaxies. We

Monthly Notices of the Royal Astronomical Society, 2007

We use a series of N-body/smoothed particle hydrodynamics simulations and analytic arguments to show that the presence of an effective temperature floor in the interstellar medium at T F ∼ 10 4 K naturally explains the tendency for low-mass galaxies to be more spheroidal, more gas rich, and less efficient in converting baryons into stars than larger galaxies. The trend arises because gas pressure support becomes important compared to angular momentum support in small dark matter haloes. We suggest that dwarf galaxies with rotational velocities ∼ 40 km s −1 do not originate as thin discs, but rather are born as thick, puffy systems. If accreted on to larger haloes, tenuous dwarfs of this kind will be more susceptible to gas loss or tidal transformation than scaled-down versions of larger spirals. For a constant temperature floor, pressure support becomes less important in large haloes, and this produces a tendency for massive isolated galaxies to have thinner discs and more efficient star formation than their less-massive counterparts, as observed.

Astronomy & Astrophysics, 2013

We present the final results from the XMM-Newton validation follow-up of new Planck galaxy cluster candidates. We observed 15 new candidates, detected with signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck survey. The candidates were selected using ancillary data flags derived from the ROSAT All Sky Survey (RASS) and Digitized Sky Survey all-sky maps, with the aim of pushing into the low SZ flux, high-z regime and testing RASS flags as indicators of candidate reliability. Fourteen new clusters were detected by XMM-Newton, ten single clusters and two double systems. Redshifts from X-ray spectroscopy lie in the range 0.2 to 0.9, with six clusters at z > 0.5. Estimated masses (M 500 ) range from 2.5 × 10 14 to 8 × 10 14 M . We discuss our results in the context of the full XMM-Newton validation programme, in which 51 new clusters have been detected. This includes four double and two triple systems, some of which are chance projections on the sky of clusters at different redshifts. We find that association with a source from the RASS-Bright Source Catalogue is a robust indicator of the reliability of a candidate, whereas association with a source from the RASS-Faint Source Catalogue does not guarantee that the SZ candidate is a bona fide cluster. Nevertheless, most Planck clusters appear in RASS maps, with a significance greater than 2σ being a good indication that the candidate is a real cluster. Candidate validation from association with SDSS galaxy overdensity at z > 0.5 is also discussed. The full sample gives a Planck sensitivity threshold of Y 500 ∼ 4 × 10 −4 arcmin 2 , with indication for Malmquist bias in the Y X -Y 500 relation below this threshold. The corresponding mass threshold depends on redshift. Systems with M 500 > 5 × 10 14 M at z > 0.5 are easily detectable with Planck. The newly-detected clusters follow the Y X -Y 500 relation derived from X-ray selected samples. Compared to X-ray selected clusters, the new SZ clusters have a lower X-ray luminosity on average for their mass. There is no indication of departure from standard self-similar evolution in the X-ray versus SZ scaling properties. In particular, there is no significant evolution of the Y X /Y 500 ratio.

Monthly Notices of the Royal Astronomical Society, 2002

We model the acquisition of spin by dark-matter halos in semi-analytic merger trees. We explore two different algorithms; one in which halo spin is acquired from the orbital angular momentum of merging satellites, and another in which halo spin is gained via tidal torquing on shells of material while still in the linear regime. We find that both scenarios produce the characteristic spin distribution of halos found in N-body simulations, namely, a log-normal distribution with mean ≈ 0.04 and standard deviation ≈ 0.5 in the log. A perfect match requires fine-tuning of two free parameters. Both algorithms also reproduce the general insensitivity of the spin distribution to halo mass, redshift and cosmology seen in N-body simulations. The spin distribution can be made strictly constant by physically motivated scalings of the free parameters. In addition, both schemes predict that halos which have had recent major mergers have systematically larger spin values. These algorithms can be implemented within semi-analytic models of galaxy formation based on merger trees. They yield detailed predictions of galaxy properties that strongly depend on angular momentum (such as size and surface brightness) as a function of merger history and environment.

Monthly Notices of the Royal Astronomical Society, 1999

We have carried out 2D hydrodynamical simulations in order to study the interaction between supernova-powered gas outflows from low-mass galaxies and the local intergalactic medium (IGM). We are specifically interested in investigating whether a high pressure IGM, such as that in clusters of galaxies, can prevent the gas from escaping from the galaxy, as suggested by Babul and Rees (1992). We find that this is indeed the case as long as ram pressure effects are negligible. The interface between the outflow and ambient IGM is demarcated by a dense expanding shell formed by the gas swept-up by the outflow. A sufficiently high IGM pressure can bring the shell to a halt well before it escapes the galaxy. Galaxies in such high pressure environments are however, more likely than not, to be ploughing through the IGM at relatively high velocities. Hence, they will also be subject to ram pressure, which acts to strip the gas from the galaxy. We have carried out simulations that take into account the combined impact of ram pressure and thermal pressure. We find that ram pressure deforms the shell into a tail-like structure, fragments it into dense clouds and eventually drags the clouds away from the galaxy. The clouds are potential sites of star formation and if viewed during this transient phase, the galaxy will appear to have a low-surface brightness tail much like the galaxies with diffuse comet-like tail seen in z=1.15 cluster 3C324. The stars in the tail would, in time, stream away from the galaxy and become part of the intracluster environment.

Monthly Notices of the Royal Astronomical Society, 2012

We discuss cosmological hydrodynamic simulations of galaxy formation performed with the new moving-mesh code AREPO, which promises higher accuracy compared with the traditional SPH technique that has been widely employed for this problem. In this exploratory study, we deliberately limit the complexity of the physical processes followed by the code for ease of comparison with previous calculations, and include only cooling of gas with a primordial composition, heating by a spatially uniform UV background, and a simple sub-resolution model for regulating star formation in the dense interstellar medium. We use an identical set of physics in corresponding simulations carried out with the well-tested SPH code GADGET, adopting also the same high-resolution gravity solver. We are thus able to compare both simulation sets on an object-by-object basis, allowing us to cleanly isolate the impact of different hydrodynamical methods on galaxy and halo properties. In accompanying papers, we focus on an analysis of the global baryonic statistics predicted by the simulation codes , and complementary idealized simulations that highlight the differences between the hydrodynamical schemes (Sijacki et al. 2011). Here we investigate their influence on the baryonic properties of simulated galaxies and their surrounding haloes. We find that AREPO leads to significantly higher star formation rates for galaxies in massive haloes and to more extended gaseous disks in galaxies, which also feature a thinner and smoother morphology than their GADGET counterparts. Consequently, galaxies formed in AREPO have larger sizes and higher specific angular momentum than their SPH correspondents. Interestingly, the more efficient cooling flows in AREPO yield higher densities and lower entropies in halo centers compared to GADGET, whereas the opposite trend is found in halo outskirts. The cooling differences leading to higher star formation rates of massive galaxies in AREPO also slightly increase the baryon content within the virial radius of massive haloes. We show that these differences persist as a function of numerical resolution. While both codes agree to acceptable accuracy on a number of baryonic properties of cosmic structures, our results thus clearly demonstrate that galaxy formation simulations greatly benefit from the use of more accurate hydrodynamical techniques such as AREPO and call into question the reliability of galaxy formation studies in a cosmological context using traditional standard formulations of SPH, such as the one implemented in GADGET. Our new moving-mesh simulations demonstrate that a population of extended gaseous disks of galaxies in large volume cosmological simulations can be formed even without energetic feedback in the form of galactic winds, although such outflows appear required to obtain realistic stellar masses.

Monthly Notices of the Royal Astronomical Society, 2012

We present the first hydrodynamical simulations of galaxy formation using the new moving mesh code AREPO and compare the results with equivalent GADGET simulations based on the traditional smoothed particle hydrodynamics (SPH) technique. Both codes use an identical Tree-PM gravity solver and include the same sub-resolution physics for the treatment of star formation, but employ a completely different method to solve the inviscid Euler equations. This allows us to cleanly assess the impact of hydro-solver uncertainties on the results of cosmological studies of galaxy formation. In this paper, we focus on predictions for global baryon statistics, such as the cosmic star formation rate density, after we introduce our simulation suite and numerical methods. Properties of individual galaxies and haloes are examined by , while a third paper by uses idealised simulations to analyse in more detail the differences between the hydrodynamical schemes. We find that the global baryon statistics differ significantly between the two simulation approaches. AREPO shows systematically higher star formation rates at late times, lower mean temperatures averaged over the simulation volume, and different gas mass fractions in characteristic phases of the intergalactic medium, in particular a reduced amount of hot gas. Although both hydrodynamical codes use the same implementation of cooling and yield an identical dark matter halo mass function, more gas cools out of haloes in AREPO compared with GADGET towards low redshifts, which induces corresponding differences in the late-time star formation rates of galaxies. We show that this striking difference originates through a higher heating rate with SPH in the outer parts of haloes, caused by viscous dissipation of SPH's inherent sonic velocity noise and SPH's efficient damping of subsonic turbulence injected in the halo infall region, and because of a higher efficiency of gas stripping in AREPO. These differences also produce more disk-like galaxy morphologies in the moving mesh calculations compared to SPH. Our results hence demonstrate that inaccuracies in hydrodynamic solvers can lead to comparatively large systematic differences even at the level of predictions for the global state of baryons in the universe.

The Astrophysical Journal, 2002

We use hydrodynamic simulations to examine how the baryonic components of galaxies are assembled, focusing on the relative importance of mergers and smooth accretion in the formation of ∼ L * systems. In our primary simulation, which models a (50h −1 Mpc) 3 comoving volume of a Λ-dominated cold dark matter universe, the space density of objects at our (64-particle) baryon mass resolution threshold M c = 5.4 × 10 10 M ⊙ corresponds to that of observed galaxies with L ∼ L * /4. Galaxies above this threshold gain most of their mass by accretion rather than by mergers. At the redshift of peak mass growth, z ≈ 2, accretion dominates over merging by about 4:1. The mean accretion rate per galaxy declines from ∼ 40M ⊙ yr −1 at z = 2 to ∼ 10M ⊙ yr −1 at z = 0, while the merging rate peaks later (z ≈ 1) and declines more slowly, so by z = 0 the ratio is about 2:1. We cannot distinguish truly smooth accretion from merging with objects below our mass resolution threshold, but extrapolating our measured mass spectrum of merging objects, dP/dM ∝ M −α with α ∼ 1, implies that sub-resolution mergers would add relatively little mass. The global star formation history in these simulations tracks the mass accretion rate rather than the merger rate. At low redshift, destruction of galaxies by mergers is approximately balanced by the growth of new systems, so the comoving space density of resolved galaxies stays nearly constant despite significant mass evolution at the galaxy-by-galaxy level. The predicted merger rate at z 1 agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift surveys.

Nature, 2005

Astronomy and Astrophysics, 2006

Aims. We report on a search for faint (R total magnitude fainter than 21) and low surface brightness galaxies (R central surface brightness fainter than ∼24) (fLSBs) in a 0.72×0.82 deg 2 area centered on the Coma cluster. Methods. We analyzed deep B and R band CCD imaging obtained using the CFH12K camera at CFHT and found 735 fLSBs. The total B magnitudes, at the Coma cluster redshift, range from −13 to −9 with B central surface brightness as faint as 27 mag arcsec −2 . Results. Using empty field comparisons, we show that most of these fLSBs are probably inside the Coma cluster. We present the results of comparing the projected fLSB distributions with the distributions of normal galaxies and with known X-ray over densities. We also investigate their projected distribution relative to their location in the color magnitude relation. Colors of fLSBs vary between B−R∼0.8 and ∼1.4 for 2/3 of the sample and this part is consistent with the known CMR red-sequence for bright (R≤18) ellipticals in Coma. Conclusions. These fLSBs are likely to have followed the same evolution as giant ellipticals, which is consistent with a simple feedback/collapse formation and a passive evolution. These fLSBs are mainly clustered around NGC 4889. We found two other distinct fLSB populations. These populations have respectively redder and bluer colors compared to the giant elliptical red-sequence and possibly formed from stripped faint ellipticals and material stripped from spiral in-falling galaxies.

The Astronomical Journal, 2007

We present H I observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H I. This H I halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M HI > ∼ 10 6 M ⊙ ) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing low angular momentum material needed to account for the observed rotation lag. The long H I filament and the counter-rotating clouds may be direct evidence of such accretion.

Nature, 2006

revealed the presence of supermassive black holes 1-4 . These black holes may interact with 1 their host galaxies by means of 'feedback' in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to the observed relations between the black hole and the host 5 . Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies.

Nature, 2005

To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs 1 . Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours 2-4 , but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited Iband survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates 2-4 , with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10-100 solar masses per year. As a consequence, the cosmic star formation rate is higher than previously measured at redshifts of 3 to 4.

Astronomy & Astrophysics, 2012

Aims. We investigate the global galaxy evolution over ∼12 Gyr (0.05 ≤ z ≤ 4.5), from the far ultraviolet (FUV) luminosity function (LF), luminosity density (LD), and star formation rate density (SFRD), using the VIMOS-VLT Deep Survey (VVDS), a single deep galaxy redshift survey with a well controlled selection function. Methods. We combine the VVDS Deep (17.5 ≤ I AB ≤ 24.0) and Ultra-Deep (23.00 ≤ i ′ AB ≤ 24.75) redshift surveys, totalizing ∼ 11000 galaxies, to estimate the rest-frame FUV LF and LD, using a wide wavelength range of deep photometry (337 < λ < 2310 nm). We extract the dust attenuation of the FUV radiation, embedded in the well-constrained spectral energy distributions. We then derive the dust-corrected SFRD. Results. We find a constant and flat faint-end slope α in the FUV LF at z < 1.7. At z > 1.7, we set α steepening with (1 + z). The absolute magnitude M * FUV steadily brightens in the entire range 0 < z < 4.5, and at z > 2 it is on average brighter than in the literature, while φ * is on average smaller. The evolution of our total LD shows a peak at z ≃ 2, clearly present also when considering all sources of uncertainty. The SFRD history peaks as well at z ≃ 2. It first steadily rises by a factor of ∼ 6 during 2 Gyr (from z = 4.5 to z = 2), and then decreases by a factor of ∼ 12 during 10 Gyr down to z = 0.05. This peak is produced by the decreasing contribution at z < 2 of galaxies with −21.5 ≤ M FUV ≤ −19.5 mag. As times goes by, the total SFRD is dominated by fainter and fainter galaxies. Moreover, at z > 2 the SFRD is entirely shaped by the high specific SFR galaxies. The mean dust attenuation of the global galaxy population rises fast by 1 mag during 2 Gyr from z ≃ 4.5 to z ∼ 2, reaches slowly its maximum at z ≃ 1 (A FUV ≃ 2.2 mag), and then decreases by 1.1 mag during 7 Gyr down to z ≃ 0. Conclusions. We have derived the cosmic SFRD history and the total dust amount in galaxies over a continuous period of ∼ 12 Gyr, using a single homogeneous spectroscopic redshift sample. The presence of a clear peak at z ≃ 2 and a fast rise at z > 2 of the SFRD is compelling for models of galaxy formation. This peak is produced by intermediate luminosity galaxies, requiring that significant gas reservoirs still exist at this epoch and are probably replenished by cold accretion and wet mergers, while feedback or quenching processes are not yet strong enough to lower the SF. The dust attenuation maximum is reached ∼2 Gyr after the SFRD peak, implying a contribution from the intermediate-mass stars to the dust production at z < 2.

The Astrophysical Journal, 2007

We present specific Star Formation Rate (sSFR) radial profiles for a sample of 161 relatively face-on spiral galaxies from the GALEX Atlas of Nearby Galaxies. The sSFR profiles are derived from GALEX & 2MASS (FUV−K) color profiles after a proper SFR calibration of the UV luminosity and K-band mass-to-light ratio are adopted. The (FUV−K) profiles were first corrected for foreground Galactic extinction and later for internal extinction using the ratio of the totalinfrared (TIR) to FUV emission. For those objects where TIR-to-FUV-ratio radial profiles were not available, the (FUV−NUV) color profiles as a measure of the UV slope. The sSFR radial gradients derived from these profiles allow us to quantify the inside-out scenario for the growth of spiral disks for the first time in the local Universe.

The Astrophysical Journal, 2004

We model how repeated supernova explosions in high-redshift dwarf starburst galaxies drive superbubbles and winds out of the galaxies. We compute the efficiencies of metal and mass ejection and energy transport from the galactic potentials, including the effect of cosmological infall of external gas. The starburst bubbles quickly blow out of small, high-redshift, galactic disks, but must compete with the ram pressure of the infalling gas to escape into intergalactic space. We show that the assumed efficiency of the star formation rate dominates the bubble evolution and the metal, mass, and energy feedback efficiencies.

The Astronomical Journal, 2011

We used the near-IR imager/spectrograph LUCIFER mounted on the Large Binocular Telescope to image, with subarcsecond seeing, the local dwarf starburst NGC 1569 in the JHK bands and He i 1.08 µm, [Fe ii]1.64µm, and Brγ narrowband filters. We obtained high-quality spatial maps of He i 1.08 µm, [Fe ii]1.64µm, and Brγ emission across the galaxy, and used them together with Hubble Space Telescope/Advanced Camera for Surveys images of NGC 1569 in the Hα filter to derive the two-dimensional spatial map of the dust extinction and surface star formation rate (SFR) density. We show that dust extinction (as derived from the Hα/Brγ flux ratio) is rather patchy and, on average, higher in the northwest (NW) portion of the galaxy (E g (B − V ) ≃ 0.71 mag) than in the southeast (E g (B − V ) ≃ 0.57 mag). Similarly, the surface density of SFR (computed from either the dereddened Hα or dereddened Brγ image) peaks in the NW region of NGC 1569, reaching a value of about 4 × 10 −6 M ⊙ yr −1 pc −2 . The total SFR as estimated from the integrated, dereddened Hα (or, alternatively, Brγ ) luminosity is about 0.4 M ⊙ yr −1 , and the total supernova rate from the integrated, dereddened [Fe ii]1.64µm luminosity is about 0.005 yr −1 (assuming a distance of 3.36 Mpc). The azimuthally averaged [Fe ii]1.64µm/Brγ flux ratio is larger at the edges of the central, gas-deficient cavities (encompassing the superstar clusters A and B) and in the galaxy outskirts. If we interpret this line ratio as the ratio between the average past star formation (as traced by supernovae) and ongoing activity (represented by OB stars able to ionize the interstellar medium), it would then indicate that star formation has been quenched within the central cavities and lately triggered in a ring around them. The number of ionizing hydrogen and helium photons as computed from the integrated, dereddened Hα and He i 1.08 µm luminosities suggests that the latest burst of star formation occurred about 4 Myr ago and produced new stars with a total mass of ≃1.8 × 10 6 M ⊙ .

Monthly Notices of the Royal Astronomical Society, 2009

In hierarchical models of structure formation, the time derivative of the halo mass function may be thought of as the difference of two terms -a creation term, which describes the increase in the number of haloes of mass m from mergers of less massive objects, and a destruction term, which describes the decrease in the number of m-haloes as these merge with other haloes, creating more massive haloes as a result. The first part of this paper focuses on estimating the distribution of times when these creation events take place. In models where haloes form from a spherical collapse, this distribution can be estimated from the same formalism which is used to estimate halo abundances: the constant-barrier excursion-set approach. In the excursion set approach, moving, rather than constant-barriers, are necessary for estimating halo abundances when the collapse is triaxial. First we generalise the excursion-set estimate of the creation time distribution by incorporating ellipsoidal collapse. Then we show that these moving-barrier based predictions are in better agreement with measurements in numerical simulations than are the corresponding predictions of the spherical collapse model. In the second part of the paper we link the creation times distribution to the creation term mentioned above. For this quantity, the improvement provided by the ellipsoidal collapse model is more evident. These results should be useful for studies of merger-driven star-formation rates and AGN activity. We also present a similar study of creation of haloes conditioned on belonging to an object of a certain mass today, and reach similar conclusions -the moving barrier based estimates are in substantially better agreement with the simulations. This part of the study may be useful for understanding the tendency for the oldest stars to exist in the most massive objects, and for star formation to only occur in lower mass objects at late times.

Monthly Notices of the Royal Astronomical Society, 2014

We present HST/COS observations of highly ionized absorption lines associated with a radioloud QSO at z = 1.1319. The absorption system has multiple velocity components, with an overall width of ≈ 600 km s −1 , tracing gas that is largely outflowing from the QSO at velocities of a few 100 km s −1 . There is an unprecedented range in ionization, with detections of H I, N III, N IV, N V, O IV, O IV*, O V, O VI, Ne VIII, Mg X, S V and Ar VIII. We estimate the total hydrogen number density from the column density ratio

Monthly Notices of the Royal Astronomical Society, 2012

During the growth of a cold dark matter halo, the direction of its spin can undergo rapid changes. These could disrupt or even destroy a stellar disc forming in the halo, possibly resulting in the generation of a bulge or spheroid. We investigate the frequency of significant changes in the orientation of the angular momentum vector of dark matter haloes ("spin flips"), and their degree of correlation with mergers. We focus on haloes of mass similar to that of the Milky Way (MW) halo at redshift z = 0 (log 10 M/ h −1 M ⊙ = 12.0 → 12.5) and consider flips in the spin of the whole halo or just its inner parts. We find that a greater fraction of major mergers are associated with large spin flips than minor mergers. However, since major mergers are rare, the vast majority (93%) of large whole-halo spin flips (θ 45 • ) coincide with small mass changes, not major mergers. The spin vector of the inner halo experiences much more frequent flips than the halo as a whole. Over their entire lifetimes (i.e. after a halo acquires half of its final mass), over 10% of halos experience a flip of at least 45 • in the spin of the entire halo and nearly 60 percent experience a flip this large in the inner halo. These numbers are reduced to 9 percent for the whole halo and 47 percent for the inner halo when we consider only haloes with no major mergers after formation. Our analysis suggests that spin flips (whose effects are not currently included in galaxy formation models) could be an important factor in the morphological transformation of disc galaxies.

Monthly Notices of the Royal Astronomical Society, 2004

We have carried out a fully sampled large area (4 • × 8 • ) 21cm H i line survey of part of the Virgo cluster using the Jodrell Bank multi-beam instrument. The survey has a sensitivity some 3 times better than the standard HIJASS and HIPASS surveys. We detect 31 galaxies, 27 of which are well known cluster members. The four new detections have been confirmed in the HIPASS data and by follow up Jodrell Bank pointed observations. One object lies behind M86, but the other 3 have no obvious optical counter parts upon inspection of the digital sky survey fields. These 3 objects were mapped at Arecibo with a smaller 3 ′ .6 HPBW and a 4 times better sensitivity than the Jodrell Bank data, which allow an improved determination of the dimensions and location of two of the objects, but surprisingly failed to detect the third. The two objects are resolved by the Arecibo beam giving them a size far larger than any optical images in the nearby field. To our mass limit of 5×10 7 ∆v 50kms −1 M ⊙ and column density limit of 3×10 18 ∆v 50kms −1 atoms cm −2 these new detections represent only about 2% of the cluster atomic hydrogen mass. Our observations indicate that the H i mass function of the cluster turns down at the low mass end making it very different to the field galaxy H i mass function. This is quite different to the Virgo cluster optical luminosity function which is much steeper than that in the general field. Many of the sample galaxies are relatively gas poor compared to H i selected samples of field galaxies, confirming the 'anaemic spirals' view of Virgo cluster late type galaxies. The velocity distribution of the H i detected galaxies is also very different to that of the cluster as a whole. There are relatively more high velocity galaxies in the H i sample, suggesting that they form part of a currently infalling population. The H i sample with optical identifications has a minimum H i column density cut-off more than an order of magnitude above that expected from the sensitivity of the survey. This observed column density is above the normally expected level for star formation to occur. The two detections with no optical counterparts have very much lower column densities than that of the rest of the sample, below the star formation threshold.

The Astrophysical Journal, 2005

We show how the observed bimodality in the color distribution of galaxies can be explained in the framework of the hierarchical clustering picture in terms of the interplay between the properties of the merging histories and the feedback/star-formation processes in the progenitors of local galaxies. Using a semi-analytic model of hierarchical galaxy formation, we compute the color distributions of galaxies with different luminosities and compare them with the observations. Our fiducial model matches the fundamental properties of the observed distributions, namely: 1) the distribution of objects brighter than M r ∼ < −18 is clearly bimodal, with a fraction of red objects increasing with luminosity; 2) for objects brighter than M r ∼ < −21 the color distribution is dominated by red objects with color u − r ≈ 2.2 − 2.4; 3) the spread on the distribution of the red population is smaller than that of the blue population; 4) the fraction of red galaxies is larger in denser environments, even for low-luminosity objects; 5) the bimodality in the distribution persists up to z ≈ 1.5. We discuss the role of the different physical processes included in the model in producing the above results.

The Astrophysical Journal, 1997

We probe to what extent not only the counts of the faint blue galaxies and their redshift distribution but also their z-resolved luminosity functions can be explained in terms of binary merging (aggregations).

Monthly Notices of the Royal Astronomical Society, 2006

Ram pressure stripping of galactic gas is generally assumed to be inefficient in galaxy groups due to the relatively low density of the intragroup medium and the small velocity dispersions of groups. To test this assumption, we obtained Chandra X-ray data of the starbursting spiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for a strong galaxy interaction with hot intragroup gas. The data reveal a shock-like feature along the western edge of the galaxy and a low-surface-brightness tail extending to the east, similar to the morphology seen in other wavebands. Spatially resolved spectroscopy shows that the data are consistent with intragroup gas being pressurized at the leading western edge of NGC 2276 due to the galaxy moving supersonically through the intragroup medium at a velocity ∼ 850 km s −1 . Detailed modelling of the gravitational potential of NGC 2276 shows that the resulting ram-pressure could significantly affect the morphology of the outer gas disc but is probably insufficient to strip large amounts of cold gas from the disc. We estimate the mass loss rates due to turbulent viscous stripping and starburst outflows being swept back by ram pressure, showing that both mechanisms could plausibly explain the presence of the X-ray tail.

Monthly Notices of the Royal Astronomical Society, 2011

We propose a simple model for the origin of fast and slow rotator early-type galaxies (ETG) within the hierarchical cold dark matter ( CDM) scenario, that is based on the assumption that the mass fraction of stellar discs in ETGs is a proxy for the specific angular momentum expressed via λ R . Within our model we reproduce the fraction of fast and slow rotators as a function of magnitude in the ATLAS 3D survey, assuming that fast-rotating ETGs have at least 10 per cent of their total stellar mass in a disc component. In agreement with ATLAS 3D observations we find that slow rotators are predominantly galaxies with M * > 10 10.5 M contributing ∼20 per cent to the overall ETG population. We show in detail that the growth histories of fast and slow rotators are different, supporting the classification of ETGs into these two categories. Slow rotators accrete between ∼50 and 90 per cent of their stellar mass from satellites and their most massive progenitors have on average up to three major mergers during their evolution. Fast rotators in contrast accrete less than 50 per cent and have on average less than one major merger in their past.

Monthly Notices of the Royal Astronomical Society, 2012

We examine the rotation rates, sizes and star formation (SF) efficiencies of a representative population of simulated disc galaxies extracted from the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC) suite of cosmological hydrodynamic simulations. These simulations include efficient, but energetically feasible supernova feedback, but have not been tuned in any way to produce 'realistic' disc galaxies. Yet, they generate a large number of discs, without requiring extremely high resolution. Over the wide galaxy stellar mass range, 9.0 log 10 [M * (M )] < 10.5, the simulations reproduce the observed Tully-Fisher relation, the rotation curves of disc galaxies in bins of stellar mass, the mass-size relation of disc galaxies, the optical rotation to virial circular velocity ratio ('V opt /V vir ') and the SF efficiencies of disc galaxies as inferred from stacked weak lensing and stacked satellite kinematics observations. They also reproduce the specific star formation rates of ∼L * galaxies but predict too low levels of SF for low-mass galaxies, which is plausibly due to the finite resolution of the simulations. At higher stellar masses, log 10 [M * (M )] > 10.6, the simulated galaxies are too concentrated and have too high SF efficiencies. We conjecture that this shortcoming reflects the neglect of feedback from accreting supermassive black holes in these simulations. We conclude that it is possible to generate a representative population of disc galaxies that reproduces many of the observed trends of local disc galaxies using standard numerical hydrodynamic techniques and a plausible implementation of the 'subgrid' astrophysical processes thought to be relevant to galaxy formation.

Physical Review D, 2010

In this work we investigate higher order statistics for the ΛCDM and ReBEL scalar-interacting dark matter models by analyzing 180 h −1 Mpc dark matter N-body simulation ensembles. The Npoint correlation functions and the related hierarchical amplitudes, such as skewness and kurtosis, are computed using the Count-In-Cells method. Our studies demonstrate that the hierarchical amplitudes Sn of the scalar-interacting dark matter model significantly deviate from the values in the ΛCDM cosmology on scales comparable and smaller then the screening length rs of a given scalar-interacting model. The corresponding additional forces that enhance the total attractive force exerted on dark matter particles at galaxy scales lowers the values of the hierarchical amplitudes Sn. We conclude that hypothetical additional exotic interactions in the dark matter sector should leave detectable markers in the higher-order correlation statistics of the density field. We focussed in detail on the redshift evolution of the dark matter field's skewness and kurtosis. From this investigation we find that the deviations from the canonical ΛCDM model introduced by the presence of the "fifth" force attain a maximum value at redshifts 0.5 < z < 2. We therefore conclude that moderate redshift data are better suited for setting observational constraints on the investigated ReBEL models. PACS numbers: 98.80.-k, 95.35.+d, 98.65.Dx

Physical review letters, 2014

The velocity field of dark matter and galaxies reflects the continued action of gravity throughout cosmic history. We show that the low-order moments of the pairwise velocity distribution v_{12} are a powerful diagnostic of the laws of gravity on cosmological scales. In particular, the projected line-of-sight galaxy pairwise velocity dispersion σ_{12}(r) is very sensitive to the presence of modified gravity. Using a set of high-resolution N-body simulations, we compute the pairwise velocity distribution and its projected line-of-sight dispersion for a class of modified gravity theories: the chameleon f(R) gravity and Galileon gravity (cubic and quartic). The velocities of dark matter halos with a wide range of masses would exhibit deviations from general relativity at the (5-10)σ level. We examine strategies for detecting these deviations in galaxy redshift and peculiar velocity surveys. If detected, this signature would be a &quot;smoking gun&quot; for modified gravity.

The Astrophysical Journal, 2006

This paper constructs an analytic framework for calculating the assembly of galactic disks from the collapse of gas within dark matter halos, with the goal of determining the resulting surface density profiles. In this formulation of the problem, gas parcels (baryons) fall through the potentials of dark matter halos on nearly ballistic, zero energy orbits and collect in a rotating disk. The dark matter halos have a nearly universal form, as determined previously through numerical simulations. The simplest scenario starts with a gaseous sphere in slow uniform rotation, follows its subsequent collapse, and determines the surface density of the disk. This calculation is carried out for pre-collapse mass distributions of the form M(r) ∼ r p with 1 ≤ p ≤ 3 and for polytropic spheres in hydrostatic equilibrium with the halo potential. The resulting disk surface density profiles have nearly power-law forms σ ∼ ̟ −q (where ̟ is the radial coordinate in the disk), with well-defined edges determined by the centrifugal barrier R C -the radius to which gas with the highest specific angular momentum falls during the collapse. This idealized scenario is generalized to include non-spherical starting states, alternate rotation profiles, and multiple accretion events (e.g., due to gas being added to the halo via merger events). This latter complication is explored in some detail and considers a log-normal distribution for the angular momenta of the pre-collapse states of the individual components. In the limit where this distribution is wide, the composite surface density approaches a universal form σ T ∼ ̟ −2 , independent of the shape of the constituent profiles. When the angular momentum distribution has an intermediate width, however, the composite surface density attains a nearly exponential form, roughly consistent with profiles of observed galaxies.

Monthly Notices of the Royal Astronomical Society, 2008

In the standard galaxy formation scenario plasma clouds with a high thermal energy content must exist at high redshifts since the proto-galactic gas is shock heated to the virial temperature and extensive cooling, leading to effcient star formation, must await the collapse of massive halos. Massive plasma clouds are potentially observable through the thermal and kinetic Sunyaev-Zel'dovich effects and their free-free emission. We found that the detection of substantial numbers of galaxyscale thermal SZ signals is achievable by blind surveys with next generation radio telescope arrays such as EVLA, ALMA and SKA. This population is even detectable with the 10% SKA, and wide field of view options at high frequency on any of these arrays would greatly increase survey speed. An analysis of confusion effects and of the contamination by radio and dust emissions shows that the optimal frequencies are those in the range 10-35 GHz.

The Astrophysical Journal, 2006

We present new relationships between halo masses (M h ) and several galaxy properties, including r * -band luminosities (L r ), stellar (M star ) and baryonic masses, stellar velocity dispersions (σ), and black hole masses (M BH ). Approximate analytic expressions are given. In the galaxy halo mass range 3 × 10 10 M ⊙ ≤ M h ≤ 3 × 10 13 M ⊙ the M h -L r , M star -M h , and M BH -M h are well represented by a double power law, with a break at M h,break ≈ 3 × 10 11 M ⊙ , corresponding to a mass in stars M star ∼ 1.2 × 10 10 M ⊙ , to a r * -band luminosity L r ∼ 5 × 10 9 L ⊙ , to a stellar velocity dispersion σ ≃ 88 km s −1 , and to a black hole mass M BH ∼ 9 × 10 6 M ⊙ . The σ-M h relation can be approximated by a single power law, though a double power law is a better representation. Although there are significant systematic errors associated to our method, the derived relationships are in good agreement with the available observational data and have comparable uncertainties. We interpret these relations in terms of the effect of feedback from supernovae and from the active nucleus on the interstellar medium. We argue that the break of the power laws occurs at a mass which marks the transition between the dominance of the stellar and the AGN feedback.

Monthly Notices of the Royal Astronomical Society, 2012

In the standard picture of disc galaxy formation, baryons and dark matter receive the same tidal torques, and therefore approximately the same initial specific angular momentum. However, observations indicate that disc galaxies typically have only about half as much specific angular momentum as their dark matter haloes. We argue this does not necessarily imply that baryons lose this much specific angular momentum as they form galaxies. It may instead indicate that galaxies are most directly related to the inner regions of their host haloes, as may be expected in a scenario where baryons in the inner parts of haloes collapse first. A limiting case is examined under the idealised assumption of perfect angular momentum conservation. Namely, we determine the density contrast ∆, with respect to the critical density of the Universe, by which dark matter haloes need to be defined in order to have the same average specific angular momentum as the galaxies they host. Under the assumption that galaxies are related to haloes via their characteristic rotation velocities, the necessary ∆ is ∼ 600. This ∆ corresponds to an average halo radius and mass which are ∼ 60% and ∼ 75%, respectively, of the virial values (i.e., for ∆ = 200). We refer to this radius as the radius of baryonic collapse R BC , since if specific angular momentum is conserved perfectly, baryons would come from within it. It is not likely a simple step function due to the complex gastrophysics involved, therefore we regard it as an effective radius. In summary, the difference between the predicted initial and the observed final specific angular momentum of galaxies, which is conventionally attributed solely to angular momentum loss, can more naturally be explained by a preference for collapse of baryons within R BC , with possibly some later angular momentum transfer.

Monthly Notices of the Royal Astronomical Society, 2011

A dark-matter-only Horizon Project simulation is used to investigate the environment-and redshift-dependence of accretion onto both halos and subhalos. These objects grow in the simulation via mergers and via accretion of diffuse non-halo material, and we measure the combined signal from these two modes of accretion. It is found that the halo accretion rate varies less strongly with redshift than predicted by the Extended Press-Schechter (EPS) formalism and is dominated by minor-merger and diffuse accretion events at z = 0, for all halos. These latter growth mechanisms may be able to drive the radio-mode feedback hypothesised for recent galaxy-formation models, and have both the correct accretion rate and form of cosmological evolution. The low redshift subhalo accretors in the simulation form a massselected subsample safely above the mass resolution limit that reside in the outer regions of their host, with ∼ 70% beyond their host's virial radius, where they are probably not being significantly stripped of mass. These subhalos accrete, on average, at higher rates than halos at low redshift and we argue that this is due to their enhanced clustering at small scales. At cluster scales, the mass accretion rate onto halos and subhalos at low redshift is found to be only weakly dependent on environment and we confirm that at z ∼ 2 halos accrete independently of their environment at all scales, as reported by other authors. By comparing our results with an observational study of black hole growth, we support previous suggestions that at z > 1, dark matter halos and their associated central black holes grew coevally, but show that by the present day, dark matter halos could be accreting at fractional rates that are up to a factor 3 − 4 higher than their associated black holes.

Monthly Notices of the Royal Astronomical Society, 2005

Using semi-analytical models of galaxy formation, the origin of boxy and disky elliptical galaxies is investigated. We find that the simple scenario, motivated by N-body simulations, in which the isophotal shape is only dependent on the mass ratio of the last major merger, is not able to reproduce the observation that the fraction of boxy and disky ellipticals depends on galaxy luminosity. The observations can however be reproduced with the following reasonable assumptions: (i) equal-mass mergers lead to boxy ellipticals and unequal-mass mergers produce disky ellipticals (as motivated by N-body simulations) (ii) major mergers between bulge-dominated galaxies result always in boxy ellipticals, independent of the mass ratio, (iii) merger remnants that subsequently accrete gas leading to a secondary stellar disk with more than 20% the total stellar fraction are always disky. This scenario indicates that the isophotal shapes of merger remnants are sensitive to the morphology of their progenitors and subsequent gas infall. Boxy and disky ellipticals can be divided into two subclasses, depending on their formation history. Boxy ellipticals are either formed by equal mass mergers of disk galaxies or by major mergers of early-type galaxies. We find that disky ellipticals are indicators of unequal-mass mergers or late gas infall. Disky ellipticals with high luminosities are preferentially 1-component systems that result from unequal mass mergers whereas low-luminosity disky ellipticals are more likely to harbour secondary disk components. In addition, the fraction of disky ellipticals with secular disk components should increase in regions with higher galaxy densities. Taking into account the conversion of cuspy cores into flat low-density cores by black hole merging we find that disky ellipticals should contain central density cusps whereas boxy ellipticals should in general be characterised by flat cores. Only rare low-luminosity boxy ellipticals, resulting from equal-mass mergers of disk galaxies could have power-law cores.

The Astrophysical Journal Supplement Series, 1999

We present an extensive study of the radiative transfer in dusty galaxies based on Monte Carlo simulations. The main output of these simulations are the attenuation curves A λ (i.e. the ratio between the observed, dust extinguished, total intensity to the intrinsic unextinguished one of the galaxy as a function of wavelength). We have explored the dependence of A λ on a conspicuous set of quantities (Hubble type, inclination, dust optical thickness, dust distribution and extinction properties) for a large wavelength interval, ranging from 1250Å to the K band, thus finally providing a comprehensive atlas of dust extinction in galaxies, which is electronically available. This study is particularly suitable for inclusion into galaxy formation evolution models and to directly interpret observational data on high redshift galaxies.

Journal of Cosmology and Astroparticle Physics, 2005

Astronomy & Astrophysics, 2012

Context. The study of the signatures of multimodality in groups and clusters of galaxies, an environment for most of the galaxies in the Universe, gives us information about the dynamical state of clusters and about merging processes, which affect the formation and evolution of galaxies, groups and clusters, and larger structures -superclusters of galaxies and the whole cosmic web. Aims. We search for the presence of substructure, a non-Gaussian, asymmetrical velocity distribution of galaxies, and large peculiar velocities of the main galaxies in clusters with at least 50 member galaxies, drawn from the SDSS DR8. Methods. We employ a number of 3D, 2D, and 1D tests to analyse the distribution of galaxies in clusters: 3D normal mixture modelling, the Dressler-Shectman test, the Anderson-Darling and Shapiro-Wilk tests, as well as the Anscombe-Glynn and the D'Agostino tests. We find the peculiar velocities of the main galaxies, and use principal component analysis to characterise our results. Results. More than 80% of the clusters in our sample have substructure according to 3D normal mixture modelling, and the Dressler-Shectman (DS) test shows substructure in about 70% of the clusters. The median value of the peculiar velocities of the main galaxies in clusters is 206 km/s (41% of the rms velocity). The velocities of galaxies in more than 20% of the clusters show significant non-Gaussianity. While multidimensional normal mixture modelling is more sensitive than the DS test in resolving substructure in the sky distribution of cluster galaxies, the DS test determines better substructure expressed as tails in the velocity distribution of galaxies (possible line-of-sight mergers). Richer, larger, and more luminous clusters have larger amount of substructure and larger (compared to the rms velocity) peculiar velocities of the main galaxies. Principal component analysis of both the substructure indicators and the physical parameters of clusters shows that galaxy clusters are complicated objects, the properties of which cannot be explained with a small number of parameters or delimited by one single test.

Monthly Notices of the Royal Astronomical Society, 2011

We present a systematic study of stellar feedback processes in simulations of disk galaxy formation. Using a dark matter halo with properties similar to the ones for the Milky Way's stellar halo, we perform a comparison of different methods of distributing energy related to feedback processes to the surrounding gas. A most promising standard model is applied to halos spanning a range of masses in order to compare the results to disk galaxy scaling relations. With few exceptions we find little or no angular momentum deficiency for our galaxies and a good agreement with the angular momentum-size relation. Our galaxies are in good agreement with the baryonic Tully-Fisher relation and the slope of the photometric Tully-Fisher relation is reproduced. We find a zero-point offset of 0.7 to 1 magnitudes, depending on the employed IMF. We also study our standard feedback model in combination with additional physical processes like a UV background, kinetic feedback, a delayed energy deposition as expected for type Ia supernovae, mass return and metal-dependent cooling. Only a combination of effects yields a real improvement of the resulting galaxy by reducing the bulge, while including metal-dependent cooling increases the bulge again. We find that in general the stellar mass fraction of our galaxies is too high. In an ad-hoc experiment we show that an removal of the bulge could reconcile this. However, the fit of the Tully-Fisher relation can only be improved by delaying the star formation, but not suppressing it completely. Our models do not seem to be efficient enough to achieve either effect. We conclude that disk formation is a complex, highly interconnected problem and we expect a solution to come from a combination of small effects.

The Astrophysical Journal, 1997

In a hierarchical cosmogony galaxies build up by continuous merging of smaller structures. At redshift three the matter content of a typical present-day galaxy is dispersed over several individual clumps embedded in sheet-like structures, often aligned along filaments. We have used hydrodynamical simulations to investigate the spatial distribution and absorption properties of metal enriched gas in such regions of ongoing galaxy formation. The metal and hydrogen absorption features produced by the collapsing structures closely resemble observed QSO absorption systems over a wide range in HI column density. Strong CII and SiIV absorption occurs for lines-of-sight passing the densest regions close to the center of the protogalactic clumps, while CIV is a good tracer of the prominent filamentary structures and OVI becomes the strongest absorption feature for lines-of-sight passing through low-density regions far away from fully-collapsed objects. The observed column density ratios of the different ionic species can be well reproduced if a mean metallicity [Z/H] =−2.5, relative abundances as found in metal-poor stars, a UV background with intensity J −22 = 3 at the Lyman limit, and either a power law spectrum (J ∝ ν −1.5 ) or the spectral shape proposed by are assumed. The observed scatter in [C/H] is about a magnitude larger than that in the simulations suggesting an inhomogeneous metal distribution. Observed and simulated Doppler parameter distributions of HI and CIV absorption lines are in good agreement indicating that shock heating due to gravitational collapse is a second important heating agent in addition to photoionization heating. The two-point correlation function of CIV lines agrees reasonably well with the observed correlation function suggesting that its high-velocity tail is caused by occasional alignments of several absorbing clumps still expanding with the Hubble flow. Both high-ionization multi-component heavy-element absorbers and damped Lyman alpha systems can arise from groups of moderately sized protogalactic clumps (M baryon ∼ 10 9 M ⊙ ). Recent detections of star forming galaxies at similar redshifts are consistent with this picture.

The Astrophysical Journal, 2000

We use cosmological N-body/gasdynamical simulations that include star formation and feedback to examine the proposal that scaling laws between the total luminosity, rotation speed, and angular momentum of disk galaxies reflect analogous correlations between the structural parameters of their surrounding dark matter halos. The numerical experiments follow the formation of galaxy-sized halos in two Cold Dark Matter dominated universes: the standard Ω = 1 CDM scenario and the currently popular ΛCDM model. We find that the slope and scatter of the I-band Tully-Fisher relation are well reproduced in the simulations, although not, as proposed in recent work, as a result of the cosmological equivalence between halo mass and circular velocity: large systematic variations in the fraction of baryons that collapse to form galaxies and in the ratio between halo and disk circular velocities are observed in our numerical experiments. The Tully-Fisher slope and scatter are recovered in this model as a direct result of the dynamical response of the halo to the assembly of the luminous component of the galaxy. We conclude that models that neglect the self-gravity of the disk and its influence on the detailed structure of the halo cannot be used to derive meaningful estimates of the scatter or slope of the Tully-Fisher relation. Our models fail, however, to match the zero-point of the Tully-Fisher relation, as well as that of the relation linking disk rotation speed and angular momentum. These failures can be traced, respectively, to the excessive central concentration of dark halos formed in the Cold Dark Matter cosmogonies we explore and to the formation of galaxy disks as the final outcome of a sequence of merger events. Disappointingly, our feedback formulation, calibrated to reproduce the empirical correlations linking star formation rate and gas surface density established by Kennicutt, has little influence on these conclusions. Agreement between model and observations appears to demand substantial revision to the Cold Dark Matter scenario or to the manner in which baryons are thought to assemble and evolve into galaxies in hierarchical universes.

The Astrophysical Journal, 1999

We use high-resolution cosmological simulations that include the effects of gasdynamics and star formation to investigate the origin of the Tully-Fisher relation in the standard Cold Dark Matter cosmogony. Stars are assumed to form in collapsing, Jeans-unstable gas clumps at a rate set by the local gas density and the dynamical/cooling timescale. The energetic feedback from stellar evolution is assumed to heat the gas surrounding regions of ongoing star formation, from where it is radiated away very rapidly. The star formation algorithm thus has little effect on the rate at which gas cools and collapses and, as a result, most galaxies form their stars very early. Luminosities are computed for each model galaxy using their full star formation histories and the latest spectrophotometric models. We find that at z = 0 the stellar mass of model galaxies is proportional to the total baryonic mass within the virial radius of their surrounding halos. Circular velocity then correlates tightly with the total luminosity of the galaxy, reflecting the equivalence between mass and circular velocity of systems identified in a cosmological context. The slope of the relation steepens slightly from the red to the blue bandpasses, and is in fairly good agreement with observations. Its scatter is small, decreasing from ∼ 0.45 mag in the U-band to ∼ 0.34 mag in the K-band. The particular cosmological model we explore here seems unable to account for the zero-point of the correlation. Model galaxies are too faint at z = 0 (by about two magnitudes) if the circular velocity at the edge of the luminous galaxy is used as an estimator of the rotation speed. The Tully-Fisher relation is brighter in the past, by about ∼ 0.7 magnitudes in the B-band at z = 1, at odds with recent observations of z ∼ 1 galaxies. We conclude that the slope and tightness of the Tully-Fisher relation can be naturally explained in hierarchical models but that its normalization and evolution depend strongly on the star formation algorithm chosen and on the cosmological parameters that determine the universal baryon fraction and the time of assembly of galaxies of different mass.

The Astrophysical Journal, 1997

We use high resolution N-body/gasdynamical simulations to investigate the effects of a photoionizing UV background on the assembly of disk galaxies in hierarchically clustering universes. We focus on the mass and rotational properties of gas that can cool to form centrifugally supported disks in dark matter halos of different mass. Photoheating can significantly reduce the amount of gas that can cool in galactic halos. Depending on the strength of the UV background field, the amount of cooled gas can be reduced by up to 50% in systems with circular speeds in the range 80-200 km s −1 . The magnitude of the effect, however, is not enough to solve the "overcooling" problem that plagues hierarchical models of galaxy formation if the UV background is chosen to be consistent with estimates based on recent observations of QSO absorption systems. Photoionization has little effect on the collapse of gas at high redshift and affects preferentially gas that is accreted at late times. Since disks form inside-out, accreting higher angular momentum gas at later times, disks formed in the presence of a UV background have spins that are even smaller than those formed in simulations that do not include the effects of photoionization. This exacerbates the angular momentum problem that afflicts hierarchical models of disk formation. We conclude that photoionization cannot provide the heating mechanism required to reconcile hierarchically clustering models with observations. Energy feedback and enrichment processes from the formation and evolution of stars must therefore be indispensable ingredients for any successful model of the formation of disk galaxies.

Monthly Notices of the Royal Astronomical Society, 2003

The Millennium Galaxy Catalogue (MGC) is a 37.5 deg 2 , medium-deep, B-band imaging survey along the celestial equator, taken with the Wide Field Camera on the Isaac Newton Telescope. The survey region is contained within the regions of both the Two Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey Early Data Release (SDSS-EDR). The survey has a uniform isophotal detection limit of 26 mag arcsec −2 and it provides a robust, well-defined catalogue of stars and galaxies in the range 16 ≤ B MGC < 24 mag.

Monthly Notices of the Royal Astronomical Society, 2011

We investigate the properties of satellite galaxies that surround isolated hosts within the redshift range 0.01 < z < 0.15, using data taken as part of the Galaxy And Mass Assembly survey. Making use of isolation and satellite criteria that take into account stellar mass estimates, we find 3 514 isolated galaxies of which 1 426 host a total of 2 998 satellites. Separating the red and blue populations of satellites and hosts, using colour-mass diagrams, we investigate the radial distribution of satellite galaxies and determine how the red fraction of satellites varies as a function of satellite mass, host mass and the projected distance from their host. Comparing the red fraction of satellites to a control sample of small neighbours at greater projected radii, we show that the increase in red fraction is primarily a function of host mass. The satellite red fraction is about 0.2 higher than the control sample for hosts with 11.0 < log 10 M * < 11.5, while the red fractions show no difference for hosts with 10.0 < log 10 M * < 10.5. For the satellites of more massive hosts the red fraction also increases as a function of decreasing projected distance. Our results suggest that the likely main mechanism for the quenching of star formation in satellites hosted by isolated galaxies is strangulation.

The Astrophysical Journal Supplement Series, 2015

Modeling large-scale sky survey observations is a key driver for the continuing development of high resolution, large-volume, cosmological simulations. We report the first results from the 'Q Continuum' cosmological N-body simulation run carried out on the GPU-accelerated supercomputer Titan. The simulation encompasses a volume of (1300 Mpc) 3 and evolves more than half a trillion particles, leading to a particle mass resolution of m p 1.5·10 8 M . At this mass resolution, the Q Continuum run is currently the largest cosmology simulation available. It enables the construction of detailed synthetic sky catalogs, encompassing different modeling methodologies, including semi-analytic modeling and sub-halo abundance matching in a large, cosmological volume. Here we describe the simulation and outputs in detail and present first results for a range of cosmological statistics, such as mass power spectra, halo mass functions, and halo mass-concentration relations for different epochs. We also provide details on challenges connected to running a simulation on almost 90% of Titan, one of the fastest supercomputers in the world, including our usage of Titan's GPU accelerators. Subject headings: methods: N-body -cosmology: large-scale structure of the universe

Monthly Notices of the Royal Astronomical Society: Letters, 2009

We use a K-selected (22.5 < K AB < 24.0) sample of dwarf galaxies (8.4 < log(M * /M ⊙ ) < 10) at 0.89<z<1.15 in the Chandra Deep Field South (CDFS) to measure their contribution to the global star-formation rate density (SFRD), as inferred from their [OII] flux. By comparing with [OII]-based studies of higher stellar mass galaxies, we robustly measure a turnover in the [OII] luminosity density at a stellar mass of M ∼ 10 10 M ⊙ . By comparison with the [OII]-based SFRD measured from the Sloan Digital Sky Survey we confirm that, while the SFRD of the lowest-mass galaxies changes very little with time, the SFRD of more massive galaxies evolves strongly, such that they dominate the SFRD at z = 1.

The Astrophysical Journal, 2003

The present-day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between "satellite accretion" and "monolithic collapse"; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity and integrals of motion phase-space projections of these stars, coupled with a series of N-body/smoothed particle hydrodynamic chemodynamical simulations, we suggest that an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.

Monthly Notices of the Royal Astronomical Society, 2013

We develop a simple evolutionary scenario for the growth of supermassive black holes (BHs), assuming growth due to accretion only, to learn about the evolution of the BH mass function from z = 3 to 0 and from it calculate the energy budgets of different modes of feedback. We tune the parameters of the model by matching the derived X-ray luminosity function (XLF) with the observed XLF of active galactic nuclei. We then calculate the amount of comoving kinetic and bolometric feedback as a function of redshift, derive a kinetic luminosity function and estimate the amount of kinetic feedback and P dV work done by classical double Fanaroff-Riley II (FR II) radio sources. We also derive the radio luminosity function for FR IIs from our synthesized population and set constraints on jet duty cycles. Around 1/6 of the jet power from FR II sources goes into P dV work done in the expanding lobes during the time the jet is on. Anti hierarchical growth of BHs is seen in our model due to addition of an amount of mass being accreted on to all BHs independent of the BH mass. The contribution to the total kinetic feedback by active galaxies in a low accretion, kinetically efficient mode is found to be the most significant at z < 1.5. FR II feedback is found to be a significant mode of feedback above redshifts z ∼ 1.5, which has not been highlighted by previous studies.

Monthly Notices of the Royal Astronomical Society, 2014

We study the interaction of galactic wind with hot halo gas using hydrodynamical simulations. We find that the outcome of this interaction depends crucially on the wind injection density and velocity. Various phases of the extraplanar media such as high velocity clouds (HVCs), outflowing clouds, and OVI regions can originate in the interaction zones of wind with the halo gas, depending on the injection velocity and density. In our simulations the size of the HVCs is of the order of 100 pc. The total mass contained in the clouds is 10 5-10 7 M ⊙ and they have a normal distribution of velocities in the galactic standard of rest frame, similar to HVCs. For high injection density and velocity, a significant number of clouds move outwards and resemble the case of cold neutral outflows. Furthermore a 10 5-10 6 K phase is formed in our simulations which has a column density ∼ 10 18 cm −2 , and resembles the observed OVI regions. The injection velocity and density are linked with the mass loading factor of the outflow, efficiency of energy injection due to supernovae and the SFR. Comparison of the predicted morphology of extraplanar gas with observations can serve as a useful diagnostic for constraining feedback efficiency of outflows.

Monthly Notices of the Royal Astronomical Society, 2011

We present a catalogue of 39 multiple-mergers found using the mergers catalogue of the Galaxy Zoo project for z < 0.1 and compare them to corresponding semianalytical galaxies from the Millennium Simulation. We estimate the (volume-limited) multi-merger fraction of the local Universe using our sample and find it to be at least two orders of magnitude less than binary-mergers -in good agreement with the simulations (especially the Munich group). We then investigate the properties of galaxies in binary-and multi-mergers (morphologies, colours, stellar masses and environment) and compare these results with those predicted by the semi-analytical galaxies. We find that multi-mergers favour galaxies with properties typical of elliptical morphologies and that this is in qualitative agreement with the models. Studies of multi-mergers thus provide an independent (and largely corroborating) test of the Millennium semi-analytical models.

Monthly Notices of the Royal Astronomical Society

We present an exploration of the expected detection of the earliest Active Galactic Nuclei (AGN) in the Universe from state-of-art galaxy formation and evolution semi-analytic models and hydro-dynamical simulations. We estimate the number and radiative characteristics of Super Massive Black Holes (SMBHs) at z ≥ 6, a redshift range that will be intensively explored by the next generation of telescopes, in particular in the radio through the Square Kilometre Array (SKA) and at high energies with ESA's Athena X-ray Observatory. We find that Athena will be able to observe over 5000 AGN/deg 2 at the Epoch of Re-ionization (EoR), 6 ≤ z ≤ 10. Similarly, for the same redshift range the models/simulations suggest that SKA will detect at least 400 AGN/deg 2. Additionally, we stress the importance of the volume of the simulation box as well as the initial physical conditions of the models/simulations on their effect on the luminosity functions (LFs) and the creation of the most massive SMBHs that we currently observe at the EoR. Furthermore, following the evolution of the accretion mode of the SMBHs in each model/simulation, we show that, while the quasar dominates over the radio mode at the EoR, detection at radio wavelengths still reaches significant numbers even at the highest redshifts. Finally, we present the effect that the radiative efficiency has on the LFs by comparing results produced with a constant value for the radiative efficiency and more complex calculations based on the spin of each SMBH.

The Astrophysical Journal, 2002

We have identified a population of 'blank field sources' (or 'blanks') among the ROSAT bright unidentified Xray sources with faint optical counterparts. The extreme X-ray over optical flux ratio of blanks is not compatible with the main classes of X-ray emitters except for extreme BL Lacertae objects. From the analysis of ROSAT archival data we found no indication of variability and evidence for only three sources, out of 16, needing absorption in excess of the Galactic value. We also found evidence for an extended nature for only one of the 5 blanks with a serendipitous HRI detection; this source (1WGA J1226.9+3332) was confirmed as a z=0.89 cluster of galaxies. Palomar images reveal the presence of a red (O− E ≥ 2) counterpart in the X-ray error circle for 6 blanks. The identification process brought to the discovery of another high z cluster of galaxies, one (possibly extreme) BL Lac, two ultraluminous X-ray sources in nearby galaxies and two apparently normal type 1 AGNs. These AGNs, together with 4 more AGN-like objects seem to form a well defined group: they present unabsorbed X-ray spectra but red Palomar counterparts. We discuss the possible explanations for the discrepancy between the X-ray and optical data, among which: a suppressed big blue bump emission, an extreme dust to gas (∼ 40 − 60 the Galactic ratio), a high redshift (z ≥ 3.5) QSO nature, an atypical dust grain size distribution and a dusty warm absorber. These AGN-like blanks seem to be the bright (and easier to study) analogs of the sources which are found in deep Chandra observations. Three more blanks have a still unknown nature.

The Astrophysical Journal, 2005

We show that the observed Velocity Dispersion Function of E/S0 galaxies matches strikingly well the distribution function of virial velocities of massive halos virializing at z ≥ 1.5, as predicted by the standard hierarchical clustering scenario in a ΛCDM cosmology, for a constant ratio σ/V vir ≃ 0.55 ± 0.05, close to the value expected at virialization if it typically occurred at z > ∼ 3. This strongly suggests that dissipative processes and later merging events had little impact on the matter density profile. Adopting the above σ/V vir ratio, the observed relationships between photometric and dynamical properties which define the fundamental plane of elliptical galaxies, such as the luminosity-σ (Faber-Jackson) and the luminosity-effective radius relations, as well as the M BH-σ relation, are nicely reproduced. Their shapes turn out to be determined by the mutual feedback of star-formation (and supernova explosions) and nuclear activity, along the lines discussed by Granato et al. (2004). To our knowledge, this is the first semi-analytic model for which simultaneous fits of the fundamental plane relations and of the epoch-dependent luminosity function of spheroidal galaxies have been presented.

The Astrophysical Journal, 1998

We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z ≤ 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.

The Astronomical Journal, 2010

We present results from the first systematic search for outlying HII regions, as part of a sample of 96 emission-line point sources (referred to as ELdots-emission-line dots) derived from the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). Our automated ELdot-finder searches SINGG narrow-band and continuum images for high equivalent width point sources outside the optical radius of the target galaxy (> 2 × r 25 in the R-band). Follow-up longslit spectroscopy and deep GALEX images (exposure time > 1000 s) distinguish outlying HII regions from background galaxies whose strong emission lines ([OIII], Hβ or [OII]) have been redshifted into the SINGG bandpass. We find that these deep GALEX images can serve as a substitute for spectroscopic follow-up because outlying HII regions separate cleanly from background galaxies in color-color space. We identify seven SINGG systems with outlying massive star formation that span a large range in Hα luminosities corresponding to a few O stars in the most nearby cases, and unresolved dwarf satellite companion galaxies in the most distant cases. Six of these seven systems feature galaxies with nearby companions or interacting galaxies. Furthermore, our results indicate that some outlying HII regions are linked to the extended-UV disks discovered by GALEX, representing emission from the most massive O stars among a more abundant population of lower mass (or older) star clusters. The overall frequency of outlying HII regions in this sample of gas-rich galaxies is 8-11% when we correct for background emission-line galaxy contamination (∼ 75% of ELdots).

Monthly Notices of the Royal Astronomical Society, 2015

We analyze a set of volume limited samples from SDSS DR12 to quantify the degree of inhomogeneity at different length scales using Shannon entropy. We find that the galaxy distributions exhibit a higher degree of inhomogeneity as compared to a Poisson point process at all length scales. Our analysis indicates that signatures of inhomogeneities in the galaxy distributions persist at least upto a length scale of 120 h −1 Mpc. The galaxy distributions appear to be homogeneous on a scale of 140 h −1 Mpc and beyond. Analyzing a set of mock galaxy samples from a semi analytic galaxy catalogue from the Millennium simulation we find a scale of transition to homogeneity at ∼ 100 h −1 Mpc.

Monthly Notices of the Royal Astronomical Society

We study the dust evolution in galaxies by implementing a detailed dust prescription in the SAGE semi-analytical model (SAM) for galaxy formation. The new model, called Dusty SAGE, follows the condensation of dust in the ejecta of Type II supernovae and asymptotic giant branch stars, grain growth in the dense molecular clouds, destruction by supernovae shocks, and the removal of dust from the interstellar medium (ISM) by star formation, reheating, inflows, and outflows. Our model successfully reproduces the observed dust mass function at redshift z = 0 and the observed scaling relations for dust across a wide range of redshifts. We find that the dust mass content in the present Universe is mainly produced via grain growth in the ISM. By contrast, in the early Universe, the primary production mechanism for dust is the condensation in stellar ejecta. The shift of the significant production channel for dust characterizes the scaling relations of dust-to-gas (DTG) and dust-to-metal (DTM...

Monthly Notices of the Royal Astronomical Society

We present the ∼800 star formation rate maps for the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey based on H α emission maps, corrected for dust attenuation via the Balmer decrement, that are included in the SAMI Public Data Release 1. We mask out spaxels contaminated by non-stellar emission using the [O III]/H β, [N II]/H α, [S II]/H α, and [O I]/H α line ratios. Using these maps, we examine the global and resolved starforming main sequences of SAMI galaxies as a function of morphology, environmental density, and stellar mass. Galaxies further below the star-forming main sequence are more likely to have flatter star formation profiles. Early-type galaxies split into two populations with similar stellar masses and central stellar mass surface densities. The main-sequence population has centrally concentrated star formation similar to late-type galaxies, while galaxies >3σ below the main sequence show significantly reduced star formation most strikingly in the nuclear regions. The split populations support a two-step quenching mechanism, wherein halo mass first cuts off the gas supply and remaining gas continues to form stars until the local stellar mass surface density can stabilize the reduced remaining fuel against further star formation. Across all morphologies, galaxies in denser environments show a decreased specific star formation rate from the outside in, supporting an environmental cause for quenching, such as ram-pressure stripping or galaxy interactions.

Astrophysical Journal - ASTROPHYS J, 2001

We study star formation history and stellar metallicity distribution in galaxies in a Lambda cold dark matter universe using a hydrodynamic cosmological simulation. Our model predicts a star formation rate declining in time exponentially from an early epoch to the present on a timescale of 6 Gyr, which is consistent with the empirical Madau plot with modest dust obscuration. Star formation in L* galaxies continues intermittently to the present, also with an exponentially declining rate on a similar timescale, whereas in small galaxies star formation ceases at an early epoch. The mean age of the extant stars decreases only slowly with increasing redshift and exceeds 1 Gyr at z=3. Normal galaxies contain stars with a wide range of metallicity and age: stars formed at z&lt;1 have metallicity of 0.1-1.0 Zsolar, while old stars take a wide range of values, from 10-6 to 3.0 Zsolar. The mean metallicity of normal galaxies is in the range 0.1-1.0 Zsolar. Dwarf galaxies that contain only old...

Monthly Notices of the Royal Astronomical Society, 2020

We study the environments of a sample of 61 extremely rare z∼1.6 Ultra-Massive Passively Evolving Galaxies (UMPEGs: stellar masses M > 10 11.5 M) which-based on clustering analysis presented in Cheema et al. (2020)-appear to be associated with very massive (M halo ∼10 14.1 h −1 M) dark matter halos that are likely to be the progenitors of z∼0 massive (Coma-and Virgo-like) galaxy clusters. We find that UMPEGs on average have fewer than one satellite galaxy with mass ratio M sat :M UMPEG ≥1:5 (i.e., M sat > ∼ 10 10.8 M) within 0.5 Mpc; the large mass gap that we observe between the typical UMPEG and its most massive satellite implies that the z∼1.6 UMPEGs assembled through major mergers. Using observed satellite counts with merger timescales from the literature, we estimate the growth rate due to mergers with mass ratio of ≥ 1:4 to be ∼13% Gyr −1 (with a ∼2× systematic uncertainty). This relatively low growth rate is unlikely to significantly affect the shape of the massive end of the stellar mass function, whose evolution must instead be driven by the quenching of new cohorts of ultra-massive star-forming galaxies. However, this growth rate is high enough that, if sustained to z∼0, the typical z∼1.6 M UMPEG = 10 11.6 M UMPEG can grow into a M ≈ 10 12 M brightest cluster galaxy (BCG) of a present-day massive galaxy cluster. Our observations favour a scenario in which our UMPEGs are main-branch progenitors of some of the present-day BCGs that have first assembled through major mergers at high redshifts and grown further through (likely minor) merging at later times.

Astrophysics and Space Science Library, 2002

Using a cosmological galactic evolutionary approach to model the Milky Way, we calculate the star formation history (SFH) of the solar neighborhood. The good agreement we obtain with the observational inferences suggests that our physical model describes accurately the long term/large spatial trends of the local and global Milky Way SFH. In this model, star formation is triggered by disk gravitational instabilities and self-regulated by an energy balance in the ISM. The drivers of the SFH are the cosmological gas infall rate and the gas surface density determined by the primordial spin parameter. A ΛCDM cosmology was used throughout.

The Astrophysical Journal, 2000

Comparing models of Simple Stellar Populations (SSP) with observed line strengths generally provides a tool to break the age-metallicity degeneracy in elliptical galaxies. Due to the wide range of Balmer line strengths observed, ellipticals have been interpreted to exhibit an appreciable scatter in age. In this paper, we analyze Composite Stellar Population models with a simple mix of an old metal-rich and an old metal-poor component. We show that these models simultaneously produce strong Balmer lines and strong metallic lines without invoking a young population. The key to this result is that our models are based on SSPs that better match the steep increase of Hβ in metal-poor globular clusters than models in the literature. Hence, the scatter of Hβ observed in cluster and luminous field elliptical galaxies can be explained by a spread in the metallicity of old stellar populations. We check our model with respect to the so-called G-dwarf problem in ellipticals. For a galaxy subsample covering a large range in 1500−V colors we demonstrate that the addition of an old metal-poor subcomponent does not invalidate other observational constraints like colors and the flux in the mid-UV.

The Astrophysical Journal, 2004

We present the detection of 34 Lyα emission-line galaxy candidates in a 80× 80 × 60 co-moving Mpc region surrounding the known z = 2.38 galaxy cluster J2143-4423. The space density of Lyα emitters is comparable to that found by Steidel et al. when targeting a cluster at redshift 3.09, but is a factor of 5.8 ± 2.5 greater than that found by field samples at similar redshifts. The distribution of these galaxy candidates contains several 5-10 Mpc scale voids. We compare our observations with mock catalogs derived from the VIRGO consortium λCDM n-body simulations. Fewer than 1% of the mock catalogues contain voids as large as we observe. Our observations thus tentatively suggest that the galaxy distribution at redshift 2.38 contains larger voids than predicted by current models. Three of the candidate galaxies and one previously discovered galaxy have the large luminosities and extended morphologies of "Lyα blobs".

Monthly Notices of the Royal Astronomical Society, 2000

We predict the internal structure and dynamics of present-day disc galaxies using galaxy evolution models within a hierarchical formation scenario. The halo mass aggregation histories, for a flat cold dark matter model with cosmological constant, were generated and used to calculate the virialization of dark matter haloes. A diversity of halo density profiles were obtained, the most typical one being close to that suggested by Navarro, Frenk & White. We modeled the way in which discs in centrifugal equilibrium are build within the evolving dark haloes, using gas accretion rates proportional to the halo mass aggregation rates, and assuming detailed angular momentum conservation. We calculated: the gravitational interactions between halo and disc-including the adiabatic contraction of the halo due to disc formation-, and the hydrodynamics, star formation, and evolution of the galaxy discs. We find that the slope and zeropoint of the Tully-Fisher (TF) relation in the infrared bands may be explained as a direct consequence of the cosmological initial conditions. This relation is almost independent of the assumed disc mass fraction, when the disc component in the rotation curve decomposition is non-negligible. Thus, the power spectrum of fluctuations can be normalized at galaxy scales through the TF relation independently of the disc mass fraction assumed. The rms scatter of the model TF relation originates mainly from the scatter in the dark halo structure and, to a minor extension, from the dispersion of the primordial spin parameter λ. The scatter obtained from our models does not disagree with the observational estimates. Our models allow us to understand why the residuals of the TF relation do not correlate significantly with disc size or surface brightness. We can also explain why low and high surface brightness galaxies have the same TF relation; the key point is the dependence of the star formation efficiency on the disc surface density. The correlations between gas fraction and surface brightness, and between scale length and V max obtained with our models agree with those observed. Discs formed within the growing haloes, where λ is assumed to be time independent, have nearly exponential surface density distributions. The shape of the rotation curves changes with disc surface brightness and is nearly flat for most cases. The rotation curve decompositions show a dominance of dark matter down to very small radii, in conflict with some observational inferences. The introduction of shallow cores in the dark halo attenuates this difficulty and produces haloes with slightly smaller rotation velocities. Other features of our galaxy models are not strongly influenced by the shallow core.

Astronomy & Astrophysics, 2012

Aims. Dissipative phenomena occurring during the orbital evolution of a dwarf satellite galaxy around a host galaxy may leave signatures in the star formation activity and signatures in the colour magnitude diagram of the galaxy stellar content. Our goal is to reach a simple and qualitative description of the these complicated phenomena. Methods. We develop an analytical and numerical technique able to study ram pressure, Kelvin-Helmholtz instability, Rayleigh-Taylor and tidal forces acting on the star formation processes in molecular clouds. We consider it together with synthetic colour magnitude diagrams techniques. Results. We developed a method to investigate the connections existing between gas consumption processes and star formation processes in the context of the two extended-body interaction with special attention to the dwarf galaxies dynamical regime.

Universe, 2021

The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. Cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. Outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source preventing runaway cooling by carving cavities and driving shocks across the medium. The AGN feedback loop is a key feature of all modern galaxy evolution models. Here we review our knowledge of the AGN feedback process in the specific context of galaxy groups. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling-heating balance. Unlike in more massive halos, the energy supplied by the central AGN to the hot intragroup medium can exceed the gravitational binding energy of halo gas particles. We report on the state-of-the-art in observations of the feedback phenomenon and in theoretical models of the heating-cooling balance in galaxy groups. We also describe how our knowledge of the AGN feedback process impacts on galaxy evolution models and on large-scale baryon distributions. Finally, we discuss how new instrumentation will answer key open questions on the topic.

Monthly Notices of the Royal Astronomical Society, 2007

We explore the behavior of the blue galaxy fraction over the redshift range 0.75 z 1.3 in the DEEP2 Survey, both for field galaxies and for galaxies in groups. The primary aim is to determine the role that groups play in driving the evolution of galaxy colour at high z. In pursuing this aim, it is essential to define a galaxy sample that does not suffer from redshift-dependent selection effects in colour-magnitude space. We develop four such samples for this study: at all redshifts considered, each one is complete in colour-magnitude space, and the selection also accounts for evolution in the galaxy luminosity function. These samples will also be useful for future evolutionary studies in DEEP2. The colour segregation observed between local group and field samples is already in place at z ∼ 1: DEEP2 groups have a significantly lower blue fraction than the field. At fixed z, there is also a correlation between blue fraction and galaxy magnitude, such that brighter galaxies are more likely to be red, both in groups and in the field. In addition, there is a negative correlation between blue fraction and group richness. In terms of evolution, the blue fraction in groups and the field remains roughly constant from z = 0.75 to z ∼ 1, but beyond this redshift the blue fraction in groups rises rapidly with z, and the group and field blue fractions become indistinguishable at z ∼ 1.3. Careful tests indicate that this effect does not arise from known systematic or selection effects. To further ensure the robustness of this result, we build on previous mock DEEP2 catalogues to develop mock catalogues that reproduce the colour-overdensity relation observed in DEEP2 and use these to test our methods. The convergence between the group and field blue fractions at z ∼ 1.3 implies that DEEP2 galaxy groups only became efficient at quenching star formation at z ∼ 2; this result is broadly consistent with other recent observations and with current models of galaxy evolution and hierarchical structure growth.

The Astrophysical Journal Supplement Series, 2015

We present an Analytic Model of Intergalactic-medium and GAlaxy evolution since the dark ages. AMIGA is in the spirit of the popular semi-analytic models of galaxy formation, although it does not use halo merger trees but interpolates halo properties in grids that are progressively built. This strategy is less memory-demanding and allows one to start the modeling at redshifts high enough and halo masses low enough to have trivial boundary conditions. The number of free parameters is minimized by making the causal connection between physical processes usually treated as independent from each other, which leads to more reliable predictions. But the strongest points of AMIGA are: i) the inclusion of molecular cooling and metal-poor, population III (Pop III) stars, with the most dramatic feedback, and ii) the accurate follow-up of the temperature and volume filling factor of neutral, singly, and doubly ionized regions, taking into account the distinct halo mass functions in those environments. We find the following general results. Massive Pop III stars determine the IGM metallicity and temperature, and the growth of spheroids and disks is self-regulated by that of massive black holes developed from the remnants of those stars. Yet, the properties of normal galaxies and active galactic nuclei appear to be quite insensitive to Pop III star properties owing to the much higher yield of ordinary stars compared to Pop III stars and the dramatic growth of MBHs when normal galaxies begin to develop, which cause the memory loss of the initial conditions. Subject headings: galaxies-galaxies: formation-dark matter-intergalactic medium 1 Even though current simulations start at z > ∼ 100, convergence of galaxy properties is only found up to z ∼ 7 for the most favorable case of relatively low resolutions (Schaye et al. 2010).

Monthly Notices of the Royal Astronomical Society, 2012

We present positions, kinematics, and the planetary nebula luminosity function (PNLF) for 35 planetary nebulae (PNe) in the nearest starburst galaxy IC10 extending out to 3 kpc from the galaxy's centre. We take advantage of the deep imaging and spectroscopic capabilities provided by the spectrograph FOCAS on the 8.2m Subaru telescope. The PN velocities were measured through the slitless-spectroscopy technique, which allows us to explore the kinematics of IC10 with high precision. Using these velocities, we conclude that there is a kinematic connection between the H i envelope located around IC10 and the galaxy's PN population. By assuming that the PNe in the central regions and in the outskirts have similar ages, our results put strong observational constraints on the past tidal interactions in the Local Group. This is so because by dating the PN central stars, we, therefore, infer the epoch of a major episode of star formation likely linked to the first encounter of the H i extended envelope with the galaxy. Our deep [O iii] images also allow us to use the PNLF to estimate a distance modulus of 24.1±0.25, which is in agreement with recent results in the literature based on other techniques.

Journal of Cosmology and Astroparticle Physics, 2014

In this paper, we study how the presence of bulge formation in galaxies influence their inner density profile, by means of an extended version of the Del Popolo (2009) semianalytical model. As in Del Popolo (2009), the model takes into account the effect of baryons adiabatic contraction, ordered and random angular momentum, dynamical friction, and adds to the previous the effect of gas cooling, star formation, supernova feedback, and reionization. Our model shows that dwarf galaxies are bulgeless, in agreement with observations showing that the large majority of them has no stellar bulges, and are characterized by a flat profile well described by a Burkert profile. We then studied the effect of a bulge, added to the cored DM halo, on the density profile. In the case of a galaxy having a mass 10 11 M ⊙ the inner density profile has a slope α ≃ 0.65, for a bulge of 4.5 × 10 9 M ⊙ , while if bulge formation is not considered, the slope would be α ≃ 0.55. If the bulge is larger, 6.5 × 10 9 M ⊙ the slope is α ≃ 0.7. In the case of a larger galaxy with 10 12 M ⊙ the slope is α ≃ 0.85, while in absence of bulge it is α ≃ 0.75. We finally study how the inner slope α changes with the bulge mass, and we find a correlation among the two quantities. The result shows that bulge formation has an important role in shaping the inner DM density profile in agreement with Inoue & Saitoh (2011) result. The result implies that going from Sc to SO Hubble type the slope is slightly steepening due to the bulge formation, and due to the fact that early type galaxies have larger bulges.

International Journal of Modern Physics D

Modified dark matter (MDM) is a phenomenological model of dark matter, inspired by gravitational thermodynamics. For an accelerating universe with positive cosmological constant ([Formula: see text]), such phenomenological considerations lead to the emergence of a critical acceleration parameter related to [Formula: see text]. Such a critical acceleration is an effective phenomenological manifestation of MDM, and it is found in correlations between dark matter and baryonic matter in galaxy rotation curves. The resulting MDM mass profiles, which are sensitive to [Formula: see text], are consistent with observational data at both the galactic and cluster scales. In particular, the same critical acceleration appears both in the galactic and cluster data fits based on MDM. Furthermore, using some robust qualitative arguments, MDM appears to work well on cosmological scales, even though quantitative studies are still lacking. Finally, we comment on certain nonlocal aspects of the quanta ...

Monthly Notices of the Royal Astronomical Society

We present JVLA-C observations of the H i gas in JO204, one of the most striking jellyfish galaxies from the GASP survey. JO204 is a massive galaxy in the low-mass cluster A957 at z = 0.04243. The H i map reveals an extended 90 kpc long ram-pressure stripped tail of neutral gas, stretching beyond the 30 kpc long ionized gas tail and pointing away from the cluster centre. The H i mass seen in emission is $(1.32\pm 0.13) \times 10^{9} \, \rm M_{\odot }$, mostly located in the tail. The northern part of the galaxy disc has retained some H i gas, while the southern part has already been completely stripped and displaced into an extended unilateral tail. Comparing the distribution and kinematics of the neutral and ionized gas in the tail indicates a highly turbulent medium. Moreover, we observe associated H i absorption against the 11 mJy central radio continuum source with an estimated H i absorption column density of 3.2 × 1020 cm−2. The absorption profile is significantly asymmetric w...

Astronomy & Astrophysics, 2016

Inferring the star-formation histories of the most massive and passive early-type galaxies at z < 0.3

Monthly Notices of the Royal Astronomical Society, 2012

We present a study of the stellar populations in two faint M31 dwarf satellites, Andromeda XI and Andromeda XIII. Using archival images from the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope (HST), we characterize the horizontal branch (HB) morphologies and the RR Lyrae (RRL) populations of these two faint dwarf satellites. Our new template light curve fitting routine (RRFIT) has been used to detect and characterize RRL populations in both galaxies. The mean periods of RRab (RR0) stars in And XI and And XIII are < P ab >=0.621 ± 0.026 (error1) ± 0.022 (error2), and < P ab >=0.648 ± 0.026 (error1) ± 0.022 (error2) respectively, where "error1" represents the standard error of the mean, while "error2" is based on our synthetic light curve simulations. The RRL populations in these galaxies show a lack of RRab stars with high amplitudes (Amp(V) > 1.0 mag) and relatively short periods (P ab ∼ 0.5 days), yet their period-V band amplitude (P-Amp(V)) relations track the relation defined by the M31 field halo RRL populations at ∼ 11 kpc from the center of M31. The metallicities of the RRab stars are calculated via a relationship between [Fe/H], Log P ab , and Amp(V). The resultant abundances ([Fe/H] AndXI = −1.75; [Fe/H] AndXIII = −1.74) are consistent with the values calculated from the RGB slope indicating that our measurements are not significantly affected by RRL evolutionary away from the zero age horizontal branch. The distance to each galaxy, based on the absolute V magnitudes of the RRab stars, is (m − M) 0,V =24.33 ± 0.05 for And XI and (m − M) 0,V =24.62 ± 0.05 for And XIII. We discuss the origins of And XI and And XIII based on a comparative analysis of the luminosity-metallicity (L-M) relation of Local Group dwarf galaxies.

Monthly Notices of the Royal Astronomical Society, 2007

In order to study the process of cooling in dark matter haloes and assess how well simple models can represent it, we run a set of radiative smoothed particle hydrodynamics (SPH) simulations of isolated haloes, with gas sitting initially in hydrostatic equilibrium within Navarro-Frenk-White potential wells. Simulations include radiative cooling and a scheme to convert highdensity cold gas particles into collisionless stars, neglecting any astrophysical source of energy feedback. After having assessed the numerical stability of the simulations, we compare the resulting evolution of the cooled mass with the predictions of the classical cooling model of White & Frenk and of the cooling model proposed in the MORGANA code of galaxy formation. We find that the classical model predicts fractions of cooled mass which, after about 2 central cooling times, are about one order of magnitude smaller than those found in simulations. Although this difference decreases with time, after 8 central cooling times, when simulations are stopped, the difference still amounts to a factor of 2-3. We ascribe this difference to the lack of validity of the assumption that a mass shell takes one cooling time, as computed on the initial conditions, to cool to very low temperature. Indeed, we find from simulations that cooling SPH particles take most time in travelling, at roughly constant temperature and increasing density, from their initial position to a central cooling region, where they quickly cool down to ∼10 4 K. We show that in this case the total cooling time is shorter than that computed on the initial conditions, as a consequence of the stronger radiative losses associated to the higher density experienced by these particles. As a consequence the mass cooling flow is stronger than that predicted by the classical model. The MORGANA model, which computes the cooling rate as an integral over the contribution of cooling shells and does not make assumptions on the time needed by shells to reach very low temperature, better agrees with the cooled mass fraction found in the simulations, especially at early times, when the density profile of the cooling gas is shallow. With the addition of the simple assumption that the increase in the radius of the cooling region is counteracted by a shrinking at the sound speed, the MORGANA model is also able to reproduce for all simulations the evolution of the cooled mass fraction to within 20-50 per cent, thereby providing a substantial improvement with respect to the classical model. Finally, we provide a very simple fitting function which accurately reproduces the cooling flow for the first ∼10 central cooling times.

Monthly Notices of the Royal Astronomical Society, 2020

We present a deep XMM−Newton observation of the extremely massive, rapidly rotating, relativistic-jet-launching spiral galaxy 2MASX J23453268−0449256. Diffuse X-ray emission from the hot gaseous halo around the galaxy is robustly detected out to a radius of 160 kpc, corresponding roughly to 35 per cent of the virial radius (≈450 kpc). We fit the X-ray emission with the standard isothermal β model, and it is found that the enclosed gas mass within 160 kpc is $1.15_{-0.24}^{+0.22} \times 10^{11} \, \rm {M}_{\odot }$. Extrapolating the gas mass profile out to the virial radius, the estimated gas mass is $8.25_{-1.77}^{+1.62} \times 10^{11} \, \rm {M}_{\odot }$, which makes up roughly 65 per cent of the total baryon mass content of the galaxy. When the stellar mass is considered and accounting for the statistical and systematic uncertainties, the baryon mass fraction within the virial radius is $0.121_{-0.043}^{+0.043}$, in agreement with the universal baryon fraction. The baryon mass f...

Journal of Cosmology and Astroparticle Physics, 2020

For the next generation of spectroscopic galaxy surveys, it is important to forecast their performances and to accurately interpret their large data sets. For this purpose, it is necessary to consistently simulate different populations of galaxies, in particular Emission Line Galaxies (ELGs), less used in the past for cosmological purposes. In this work, we further the forward modeling approach presented in Fagioli et al. 2018, by extending the spectra simulator Uspec to model galaxies of different kinds with improved parameters from Tortorelli et al. 2020. Furthermore, we improve the modeling of the selection function by using the image simulator Ufig. We apply this to the Sloan Digital Sky Survey (SDSS), and simulate ∼ 157, 000 multi-band images. We pre-process and analyse them to apply cuts for target selection, and finally simulate SDSS/BOSS DR14 galaxy spectra. We compute photometric, astrometric and spectroscopic properties for red and blue, real and simulated galaxies, finding very good agreement. We compare the statistical properties of the samples by decomposing them with Principal Component Analysis (PCA). We find very good agreement for red galaxies and a good, but less pronounced one, for blue galaxies, as expected given the known difficulty of simulating those. Finally, we derive stellar population properties, mass-tolight ratios, ages and metallicities, for all samples, finding again very good agreement. This shows how this method can be used not only to forecast cosmology surveys, but it is also able to provide insights into studies of galaxy formation and evolution.

Monthly Notices of the Royal Astronomical Society, 2021

The European Space Agency (ESA) Gaia mission has enabled the remarkable discovery that a large fraction of the stars near the solar neighbourhood are debris from a single in-falling system, the so-called Gaia-Sausage-Enceladus (GSE). This discovery provides astronomers for the first time with a large cohort of easily observable, unevolved stars that formed in a single extragalactic environment. Here we use these stars to investigate the ‘Spite plateau’ – the near-constant lithium abundance observed in unevolved metal-poor stars across a wide range of metallicities (−3 &lt; [Fe/H] &lt; −1). Our aim is to test whether individual galaxies could have different Spite plateaus – e.g. the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We identified 93 GSE dwarf stars observed and analysed by the GALactic Archaeology with HERMES (GALAH) survey as part of its Data Release 3 (DR3). Orbital actions were used to...

Astronomy and Astrophysics, 2006

We constrain the evolution of the galaxy mass and luminosity functions from the analysis of (public) multi-wavelength data in the Chandra Deep Field South (CDFS) area, obtained from the GOODS and other projects, and including very deep high-resolution imaging by HST/ACS. Our reference catalogue of faint high-redshift galaxies, which we have thoroughly tested for completeness and reliability, comes from a deep (S3.6 ≥ 1 µJy) image by IRAC on the Spitzer Observatory. These imaging data in the field are complemented with extensive optical spectroscopy by the ESO VLT/FORS2 and VIMOS spectrographs, while deep K-band VLT/ISAAC imaging is also used to derive further complementary statistical constraints and to assist the source identification and SED analysis. We have selected a highly reliable IRAC 3.6µm sub-sample of 1478 galaxies with S3.6 ≥ 10 µJy, 47% of which have spectroscopic redshift, while for the remaining objects both COMBO-17 and Hyperz are used to estimate the photometric redshift. This very extensive dataset is exploited to assess evolutionary effects in the galaxy luminosity and stellar mass functions, while luminosity/density evolution is further constrained with the number counts and redshift distributions. The deep ACS imaging allows us to differentiate these evolutionary paths by morphological type, which our simulations show to be reliable at least up to z ∼ 1.5 for the two main early-(E/S0) and late-type (Sp/Irr) classes. These data, as well as our direct estimate of the stellar mass function above M * h 2 = 10 10 M⊙ for the spheroidal subclass, consistently evidence a progressive dearth of such objects to occur starting at z ∼ 0.7, paralleled by an increase in luminosity. A similar trend, with a more modest decrease of the mass function, is also shared by spiral galaxies, while the irregulars/mergers show an increased incidence at higher z. Remarkably, this decrease of the comoving density with redshift of the total population appears to depend on galaxy mass, being stronger for moderate-mass, but almost absent until z = 1.4 for high-mass galaxies, thus confirming previous evidence for a "downsizing" effect in galaxy formation. Our favoured interpretation of the evolutionary trends for the two galaxy categories is that of a progressive morphological transformation (due to gas exhaustion and, likely, merging) from the star-forming to the passively evolving phase, starting at z ≥ 2 and keeping on down to z ∼ 0.7. The rate of this process appears to depend on galaxy mass, being already largely settled by z ∼ 1 for the most massive systems.

The Astrophysical Journal, 2022

We introduce a simple entropy-based formalism to characterize the role of mixing in pressure-balanced multiphase clouds and demonstrate example applications using enzo-e (magneto)hydrodynamic simulations. Under this formalism, the high-dimensional description of the system’s state at a given time is simplified to the joint distribution of mass over pressure (P) and entropy (K = P ρ −γ ). As a result, this approach provides a way to (empirically and analytically) quantify the impact of different initial conditions and sets of physics on the system evolution. We find that mixing predominantly alters the distribution along the K direction and illustrate how the formalism can be used to model mixing and cooling for fluid elements originating in the cloud. We further confirm and generalize a previously suggested criterion for cloud growth in the presence of radiative cooling and demonstrate that the shape of the cooling curve, particularly at the low-temperature end, can play an importan...

Publications of the Astronomical Society of Australia, 2003

The primary present-day observables upon which theories of galaxy evolution are based are a system’s morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.

Astronomy & Astrophysics, 2021

Context. The processes responsible for galaxy evolution in different environments as a function of galaxy mass remain heavily debated. Rich galaxy clusters are ideal laboratories in which to distinguish the role of environmental versus mass quenching because they consist of a full range of galaxies and environments. Aims. Using the CLASH-VLT survey, we assembled an unprecedentedly large sample of 1234 spectroscopically confirmed members in Abell S1063. We found a dynamically complex structure at ⟨zcl⟩ = 0.3457 with a velocity dispersion σv = 1380−32+26 km s−1. We investigated cluster environmental and dynamical effects by analysing the projected phase-space diagram and the orbits as a function of galaxy spectral properties. Methods. We classified cluster galaxies according to the presence and strength of the [OII] emission line, the strength of the Hδ absorption line, and colours. We investigated the relation between the spectral classes of galaxies and their position in the project...

The Astrophysical Journal Supplement Series, 2018

The use of high-quality simulated sky catalogs is essential for the success of cosmological surveys. The catalogs have diverse applications, such as investigating signatures of fundamental physics in cosmological observables, understanding the effect of systematic uncertainties on measured signals and testing mitigation strategies for reducing these uncertainties, aiding analysis pipeline development and testing, and survey strategy optimization. The list of applications is growing with improvements in the quality of the catalogs and the details that they can provide. Given the importance of simulated catalogs, it is critical to provide rigorous validation protocols that enable both catalog providers and users to assess the quality of the catalogs in a straightforward and comprehensive way. For this purpose, we have developed the DESCQA framework for the Large Synoptic Survey Telescope Dark Energy Science Collaboration as well as for the broader community. The goal of DESCQA is to enable the inspection, validation, and comparison of an inhomogeneous set of synthetic catalogs via the provision of a common interface within an automated framework. In this paper, we present the design concept and first implementation of DESCQA. In order to establish and demonstrate its full functionality we use a set of interim catalogs and validation tests. We highlight several important aspects, both technical and scientific, that require thoughtful consideration when designing a validation framework, including validation metrics and how these metrics impose requirements on the synthetic sky catalogs.

Monthly Notices of the Royal Astronomical Society, 2001

We present predictions for the reionization of the intergalactic medium (IGM) by stars in high-redshift galaxies, based on a semi-analytic model of galaxy formation. We calculate ionizing luminosities of galaxies, including the effects of absorption by interstellar gas and dust on the escape fraction f esc , and follow the propagation of the ionization fronts around each galaxy in order to calculate the filling factor of ionized hydrogen in the IGM. For a ΛCDM cosmology, with parameters of the galaxy formation model chosen to match observations of present-day galaxies, and a physical calculation of the escape fraction, we find that the hydrogen in the IGM will be reionized at redshift z = 6.1 if the IGM has uniform density, but only by z = 4.5 if the IGM is clumped. If instead we assume a constant escape fraction of 20% for all galaxies, then we find reionization at z = 9.0 and z = 7.8 for the same two assumptions about IGM clumping. We combine our semi-analytic model with an N-body simulation of the distribution of dark matter in the universe in order to calculate the evolution of the spatial and velocity distribution of the ionized gas in the IGM, and use this to calculate the secondary temperature anisotropies induced in the cosmic microwave background (CMB) by scattering off free electrons. The models predict a spectrum of secondary anisotropies covering a broad range of angular scales, with fractional temperature fluctuations ∼ 10 −7 − 10 −6 on arcminute scales. The amplitude depends strongly on the total baryon density, and less sensitively on the escape fraction f esc. The amplitude also depends somewhat on the geometry of reionization, with models in which the regions of highest gas density are reionized first giving larger CMB fluctuations than the case where galaxies ionize surrounding spherical regions, and models where low density regions reionize first giving the smallest fluctuations. Measurement of these anisotropies can therefore put important constraints on the reionization process, in particular, the redshift evolution of the filling factor, and should be a primary objective of a next generation submillimeter telescope such as the Atacama Large Millimeter Array.

The Astrophysical Journal, 2020

The connection between galaxy star formation rate (SFR) and dark matter (DM) is of paramount importance for the extraction of cosmological information from next generation spectroscopic surveys that will target emission line star forming galaxies. Using publicly available mock galaxy catalogs obtained from various semianalytic models (SAMs) we explore the SFR-DM connection in relation to the speed-from-light method (Feix et al. 2017) for inferring the growth rate, f , from luminosity/SFR shifts. Emphasis is given to the dependence of the SFR distribution on the environment density on scales of 10s-100s Mpc. We show that the application of the speed-from-light method to an Euclid-like survey is not biased by environmental effects. In all models, the precision on the measured β = f /b parameter is σ β < ∼ 0.17 at z = 1. This translates into errors of σ f ∼ 0.22 and σ (f σ8) ∼ 0.1, without invoking assumptions on the mass power spectrum. These errors are in the same ballpark as recent analyses of the redshift space distortions in galaxy clustering. In agreement with previous studies, the bias factor, b is roughly a scale-independent, constant function of the SFR for star forming galaxies. Its value at z = 1 ranges from 1.2 to 1.5 depending on the SAM recipe. Although in all SAMs denser environments host galaxies with higher stellar masses, the dependence of the SFR on the environment is more involved. In most models the SFR probability distribution is skewed to larger values in denser regions. One model exhibits an inverted trend where high SFR is suppressed in dense environment.

Nature Astronomy, 2019

The Astrophysical Journal, 2022

In the next decade, deep galaxy surveys from telescopes such as the James Webb Space Telescope and Roman Space Telescope will provide transformational data sets that will greatly enhance the understanding of galaxy formation during the epoch of reionization (EoR). In this work, we present the Deep Realistic Extragalactic Model (DREaM) for creating synthetic galaxy catalogs. Our model combines dark matter simulations, subhalo abundance matching and empirical models, and includes galaxy positions, morphologies, and spectral energy distributions. The resulting synthetic catalog extends to redshifts z ∼ 12, and galaxy masses log 10 ( M / M ⊙ ) = 5 covering an area of 1 deg2 on the sky. We use DREaM to explore the science returns of a 1 deg2 Roman ultra-deep field (UDF), and to provide a resource for optimizing ultra-deep survey designs. We find that a Roman UDF to ∼30 m AB will potentially detect more than 106 M UV &lt; − 17 galaxies, with more than 104 at redshifts z &gt; 7, offering a...

Monthly Notices of the Royal Astronomical Society, 2013

We analyse the optical spectra of massive (log M * /M > 11.4) radio-loud galaxies at z ∼ 0.2 and z ∼ 0.6. Our samples are generated by cross-matching the Sloan Digital Sky Survey DR7 and Baryon Oscillation Spectroscopic Survey spectroscopic galaxy catalogues with the Faint Images of the Radio Sky at Twenty centimetres and NVSS radio continuum surveys. By comparing stellar population parameters of these radio-loud samples with radio-quiet control samples matched in stellar mass, velocity dispersion and redshift, we investigate how the presence of a radio-emitting jet relates to the recent star formation history of the host galaxy. We also investigate how the emission-line properties of the radio galaxies evolve with redshift by stacking their spectra. Our main results are the following. (1) Both at low and at high redshift, half as many radio-loud as radio-quiet galaxies have experienced significant star formation in the past Gyr. This difference in star formation history is independent of the luminosity of the radio AGN, except at radio luminosities greater than 10 25.5 W Hz −1 , where it disappears. (2) The Balmer absorption-line properties of massive galaxies that have experienced recent star formation show that star formation occurred as a burst in many of these systems. (3) Both the radio and the emission-line luminosity of radio AGN evolve significantly with redshift. The average [O III] rest equivalent width increases by 1 dex from z = 0.2 to z = 0.6, and emission-line ratios change from LINER-like at low redshift to Seyfert-like at high redshift. However, radio galaxies with similar stellar population parameters have similar emission-line properties both at high and at low redshift. These results suggest that massive galaxies experience cyclical episodes of gas accretion, star formation and black hole growth, followed by the production of a radio jet that shuts down further activity. The behaviour of galaxies with log M * /M > 11.4 is the same at z = 0.6 as it is at z = 0.2, except that higher redshift galaxies experience more star formation and black hole growth and produce more luminous radio jets during each accretion cycle.

Monthly Notices of the Royal Astronomical Society, 2021

The presence of stars on retrograde orbits in disc galaxies is usually attributed to accretion events, both via direct accretion, and through the heating of the disc stars. Recent studies have shown that retrograde orbits can also be produced via scattering by dense clumps, which are often present in the early stages of a galaxy’s evolution. However, so far it has been unclear whether other internally driven mechanisms, such as bars, are also capable of driving retrograde motion. Therefore, in this paper, we investigate the efficiencies with which bars and clumps produce retrograde orbits in disc galaxies. We do this by comparing the retrograde fractions and the spatial distributions of the retrograde populations in four N-body+smooth particle hydrodynamics (SPH) simulations of isolated disc galaxies spanning a range of evolutionary behaviours. We find that both bars and clumps are capable of generating significant retrograde populations of order ${\sim}10{{\ \rm per\ cent}}$ of all...

The Astrophysical Journal, 2020

We study the evolution in the number density of galaxies at the highest stellar masses over the past ≈9 Gyr (< < z 0.4 3 (±0.05) from z=1 to z=0.4, consistent with no evolution, then declines to n log Mpc 3 =−3.7 (±0.05) at z=1.5. We discuss the uncertainties in the derived SMF, which are dominated by assumptions in the star formation history and details of stellar population synthesis models for stellar mass estimations. We also study the evolution in the SMF for samples of star-forming and quiescent galaxies selected by their specific star formation rate. For quiescent galaxies, the data are consistent with no (or slight) evolution (0.1 dex) in either the characteristic mass or number density from z∼1.5 to the present even after accounting for the systematic uncertainty and the random error in the stellar mass measurement. The lack of number density evolution in the quiescent massive galaxy population means that any mass growth (presumably through "dry" mergers) must balance the rate of stellar mass losses owing to processes of late-stage stellar evolution and the formation of newly quiescent galaxies from the star-forming population. We provide an upper limit on this mass growth from z=1.0 to 0.4 of ΔM * /M * =45% (i.e., ;0.16 dex) for quiescent galaxies more massive than 10 11 M e .

Monthly Notices of the Royal Astronomical Society, 2020

We use the improved IllustrisTNG300 magnetohydrodynamical cosmological simulation to revisit the effect that secondary halo bias has on the clustering of the central galaxy population. With a side length of 205 h−1 Mpc and significant improvements on the subgrid model with respect to previous Illustris simulations, IllustrisTNG300 allows us to explore the dependencies of galaxy clustering over a large cosmological volume and halo mass range. We show at high statistical significance that the halo assembly bias signal (i.e. the secondary dependence of halo bias on halo formation redshift) manifests itself on the clustering of the galaxy population when this is split by stellar mass, colour, specific star formation rate, and surface density. A significant signal is also found for galaxy size: at fixed halo mass, larger galaxies are more tightly clustered than smaller galaxies. This effect, in contrast to the rest of the dependencies, seems to be uncorrelated with halo formation time, w...

Astronomy &amp; Astrophysics, 2021

The spin, or normalized angular momentum λ, of dark matter halos in cosmological simulations follows a log normal distribution and has little correlation with galaxy observables such as stellar masses or sizes. There is currently no way to infer the λ parameter of individual halos hosting observed galaxies. Here, we present a first attempt to measure λ starting from the dynamically distinct disks and stellar halos identified in high-resolution cosmological simulations with the Galactic Structure Finder (gsf). In a subsample of NIHAO galaxies analyzed with gsf, we find tight correlations between the total angular momentum of the dark matter halos, Jh, and the azimuthal angular momentum, Jz, of the dynamical distinct stellar components of the form: log(Jh) = α + β⋅log(Jz). The stellar halos have the tightest relation with α = 9.50 ± 0.42 and β = 0.46 ± 0.04. The other tight relation is with the disks, for which α = 6.15 ± 0.92 and β = 0.68 ± 0.07. While the angular momentum is difficu...

Astronomy &amp; Astrophysics, 2017

Context. The galaxy M 49 (NGC 4472) is the brightest early-type galaxy in the Virgo Cluster. It is located in subcluster B and has an unusually blue, metal-poor outer halo. Planetary nebulae (PNe) are excellent tracers of diffuse galaxy and intragroup light (IGL). Aims. We aim to present a photometric survey of PNe in the galaxy's extended halo to characterise its PN population, as well as the surrounding IGL of the subcluster B. Methods. PNe were identified based on their bright [OIII]5007 Å emission and absence of a broad-band continuum through automated detection techniques. Results. We identify 738 PNe out to a radius of ∼155 kpc from M 49's centre from which we define a complete sample of 624 PNe within a limiting magnitude of m 5007,lim = 28.8. Comparing the PN number density to the broad-band stellar surface brightness profile, we find a variation of the PN-specific frequency (α-parameter) with radius. The outer halo beyond 60 kpc has a 3.2 times higher α-parameter compared to the main galaxy halo (α M 49 2.5,inner = (3.20 ± 0.43) × 10 −9 PN L −1 ,bol), which is likely due to contribution from the surrounding blue IGL. We use the planetary nebulae luminosity function (PNLF) as an indicator of distance and stellar population. Its slope, which correlates empirically with galaxy type, varies within the inner halo. In the eastern quadrant of M 49, the PNLF slope is shallower, indicating an additional localised, bright PN population following an accretion event, likely that of the dwarf irregular galaxy VCC1249. We also determined a distance modulus of µ PNLF = 31.29 +0.07 −0.08 for M 49, corresponding to a physical distance of 18.1 ± 0.6 Mpc, which agrees with a recent surface-brightness fluctuations distance. Conclusions. The PN populations in the outer halo of M 49 are consistent with the presence of a main Sérsic galaxy halo with a slight (B − V) colour gradient of 10 −4 mag arcsec −1 surrounded by IGL with a very blue colour of (B − V) = 0.25 and a constant surface brightness µ V = 28.0 mag arcsec −2 .

The Astrophysical Journal, 2020

Whereas cold dark matter (CDM) simulations predict central dark matter cusps with densities that diverge as ρ(r)∼ 1/r observations often indicate constant density cores with finite central densities ρ 0 and a flat density distribution within a core radius r 0. This paper investigates whether this core-cusp problem can be solved by fuzzy dark matter (FDM), a hypothetical particle with a mass of order m≈10 −22 eV and a corresponding de Broglie wavelength on astrophysical scales. We show that galaxies with CDM halo virial masses M vir ≤ 10 11 M ⊙ follow two core scaling relations. In addition to the well known universal core column density Σ 0 ≡ ρ 0 ×r 0 = 75 M ⊙ pc −2 core radii increase with virial masses as r 0 ∼ M γ vir with γ of order unity. Using the simulations by Schive et al. (2014) we demonstrate that FDM can explain the r 0-M vir scaling relation if the virial masses of the observed galaxy sample scale with formation redshift z as M vir ∼(1+z) −0.4. The observed constant Σ 0 is however in complete disagreement with FDM cores which are characterised by a steep dependence Σ 0 ∼r −3 0 , independent of z. More highresolution simulations are now required to confirm the Schive et al. simulations and explore especially the transition region between the soliton core and the surrounding halo. If these results hold, FDM can be ruled out as the origin of observed dark matter cores and other physical processes are required to account for their formation.

Journal of Cosmology and Astroparticle Physics, 2014

We study, by means of the model proposed in Del Popolo (2009), the effect of baryon physics on the small scale problems of the CDM model. We show that, using this model, the cusp/core problem, the missing satellite problem (MSP), the Too Big to Fail (TBTF) problem, and the angular momentum catastrophe can be reconciled with observations. Concerning the cusp/core problem, the interaction among dark matter (DM) and baryonic clumps of 1% the mass of the halo, through dynamical friction (DF), is able to flatten the inner cusp of the density profiles. We moreover assume that haloes form primarily through quiescent accretion, in agreement with the spherical collapse model (SCM)-secondary infall model (SIM) prescriptions. The results of this paper follow from the two assumptions above. Concerning the MSP and TBTF problem, applying to the Via Lactea II (VL2) subhaloes a series of corrections similar to those of Brooks et al. (2013), namely applying a Zolotov et al. (2012)-like correction obtained with our model, and further correcting for the UV heating and tidal stripping, we obtain that the number of massive, luminous satellites is in agreement with the number observed in the MW. The model also produces an angular momentum distribution in agreement with observations, that is with the distribution of the angular spin parameter and angular momentum of the dwarfs studied by van den Bosch, Burkert, & Swaters (2001). In conclusion, the small scale problems of the CDM model can all be solved by introducing baryon physics.

Galaxies, 2017

Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the ΛCDM model. Such observations are not trivial because of the extreme faintness of such regions. In this work, we investigated the photometric properties of six massive early-type galaxies (ETGs) in the VST Elliptical GAlaxies Survey (VEGAS) sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to extremely low surface brightness levels with the goal of characterizing the global structure of their light profiles for comparison to state-of-the-art galaxy formation models. We carried out deep and detailed photometric mapping of our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and i bands. By fitting the light profiles, and comparing the results to simulations of elliptical galaxy assembly, we have identified signatures of a transition between relaxed and unrelaxed accreted components and can constrain the balance between in situ and accreted stars. The very good agreement of our results with predictions from theoretical simulations demonstrates that the full VEGAS sample of ∼ 100 ETGs will allow us to use the distribution of diffuse light as a robust statistical probe of the hierarchical assembly of massive galaxies.

The Astronomical Journal, 1999

The star formation history in IZw18 has been inferred from HST/WFPC2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5-1 Gyr, at a rate of ∼ 1-2 × 10 −2 M ⊙ yr −1 kpc −2. A more intense activity of 6-16 × 10 −2 M ⊙ yr −1 kpc −2 has taken place between 15 and 20 Myr ago. For the secondary body, the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is 3-10 × 10 −3 M ⊙ yr −1 kpc −2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at α ≃ 2.2, closer to Salpeter's slope (α=2.35).

Astrophysics and Space Science Library, 2017

The properties and star formation processes in the far-outer disks of nearby spiral and dwarf irregular galaxies are reviewed. The origin and structure of the generally exponential profiles in stellar disks is considered to result from cosmological infall combined with a non-linear star formation law and a history of stellar migration and scattering from spirals, bars, and random collisions with interstellar clouds. In both spirals and dwarfs, the far-outer disks tend to be older, redder and thicker than the inner disks, with the overall radial profiles suggesting inside-out star formation plus stellar scattering in spirals, and outside-in star formation with a possible contribution from scattering in dwarfs. Dwarf irregulars and the far-outer parts of spirals both tend to be gas dominated, and the gas radial profile is often non-exponential although still decreasing with radius. The ratio of Hα to far-UV flux tends to decrease with lower surface brightness in these regions, suggesting either a change in the initial stellar mass function or the sampling of that function, or a possible loss of Hα photons.

The Astrophysical Journal Supplement Series, 2016

This paper describes a new publicly available codebase for modeling galaxy formation in a cosmological context, the "Semi-Analytic Galaxy Evolution" model, or SAGE for short. 5 SAGE is a significant update to the 2006 model of Croton et al. and has been rebuilt to be modular and customizable. The model will run on any N-body simulation whose trees are organized in a supported format and contain a minimum set of basic halo properties. In this work, we present the baryonic prescriptions implemented in SAGE to describe the formation and evolution of galaxies, and their calibration for three N-body simulations: Millennium, Bolshoi, and GiggleZ. Updated physics include the following: gas accretion, ejection due to feedback, and reincorporation via the galactic fountain; a new gas cooling-radio mode active galactic nucleus (AGN) heating cycle; AGN feedback in the quasar mode; a new treatment of gas in satellite galaxies; and galaxy mergers, disruption, and the build-up of intra-cluster stars. Throughout, we show the results of a common default parameterization on each simulation, with a focus on the local galaxy population.

The Astrophysical Journal Supplement Series, 2016

We introduce the Theoretical Astrophysical Observatory (TAO), an online virtual laboratory that houses mock observations of galaxy survey data. Such mocks have become an integral part of the modern analysis pipeline. However, building them requires an expert knowledge of galaxy modelling and simulation techniques, significant investment in software development, and access to high performance computing. These requirements make it difficult for a small research team or individual to quickly build a mock catalogue suited to their needs. To address this TAO offers access to multiple cosmological simulations and semi-analytic galaxy formation models from an intuitive and clean web interface. Results can be funnelled through science modules and sent to a dedicated supercomputer for further processing and manipulation. These modules include the ability to (1) construct custom observer light-cones from the simulation data cubes; (2) generate the stellar emission from star formation histories, apply dust extinction, and compute absolute and/or apparent magnitudes; and (3) produce mock images of the sky. All of TAO's features can be accessed without any programming requirements. The modular nature of TAO opens it up for further expansion in the future.

Astronomy & Astrophysics, 2009

Context. Environmental effects are known to have an important influence on cluster galaxies, but studies at very faint magnitudes (R > 21) are almost exclusively based on imaging. We present here a very deep spectroscopic survey of galaxies on the line of sight to the Coma cluster. Aims. After a series of papers based on deep multi-band imaging of the Coma cluster, we explore spectroscopically part of the central regions of Coma, in order to confirm and generalize previous results, concerning in particular the galaxy luminosity function, red sequence, stellar populations and the most likely formation scenario for the Coma cluster. Methods. We have obtained reliable VIMOS redshifts for 715 galaxies in the direction of the Coma cluster centre in the unprecedented magnitude range 21 ≤ R ≤ 23, corresponding to the absolute magnitude range −14 ≤ M R ≤ −12. Results. We confirm the substructures previously identified in Coma, and identify three new substructures. We detect a large number of groups behind Coma, in particular a large structure at z ∼ 0.5, the SDSS Great Wall, and a large and very young previously unknown structure at z ∼ 0.054, which we named the background massive group (BMG). These structures account for the mass maps derived from a recent weak lensing analysis. The orbits of dwarf galaxies are probably anisotropic and radial, and could originate from field galaxies radially falling into the cluster along the numerous cosmological filaments surrounding Coma. Spectral characteristics of Coma dwarf galaxies show that red or absorption line galaxies have larger stellar masses and are older than blue or emission line galaxies. R ≤ 22 galaxies show less prominent absorption lines than R ≥ 22 galaxies. This trend is less clear for field galaxies, which are similar to R ≥ 22 Coma galaxies. This suggests that part of the faint Coma galaxies could have been recently injected from the field following the NGC 4911 group infall. We present a list of five ultra compact dwarf galaxy candidates which need to be confirmed with high spatial resolution imaging with the HST. We also globally spectroscopically confirm our previous results concerning the galaxy luminosity functions based on imaging down to R = 23 (M R = −12) and find that dwarf galaxies follow a red sequence similar to that drawn by bright Coma galaxies. Conclusions. Spectroscopy of faint galaxies in Coma confirms that dwarf galaxies are very abundant in this cluster, and that they are partly field galaxies that have fallen onto the cluster along cosmological filaments.

Monthly Notices of the Royal Astronomical Society, 2014

We use a combination of observations and simulation to study the relationship between starforming galaxies and the intergalactic medium at z ≈ 3. The observed star-forming galaxy sample is based on spectroscopic redshift data taken from a combination of Very Large Telescope (VLT) Lyman-break galaxy (LBG) Redshift Survey (VLRS) data and Keck Low-Resolution Imaging Spectrometer (LRIS) observations in fields centred on bright background quasi-stellar objects (QSOs), whilst the simulation data is taken from the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC). In the simulation, we find that the dominant peculiar velocities are in the form of large-scale coherent motions of gas and galaxies. Gravitational infall of galaxies towards one another is also seen, consistent with expectations from linear theory. At smaller scales, the root-mean-square (RMS) peculiar velocities in the simulation overpredict the difference between the simulated real-and z-space galaxy correlation functions. Peculiar velocity pairs with separations smaller than 1 h −1 Mpc have a smaller dispersion and explain the z-space correlation function better. The Lyα auto-and cross-correlation functions in the GIMIC simulation appear to show infall smaller than implied by the expected β Lyα ≈ 1.3 (McDonald et al.). There is a possibility that the reduced infall may be due to the galaxy-wide outflows implemented in the simulation. The main challenge in comparing these simulated results with the observed Keck + VLRS correlation functions comes from the presence of velocity errors for the observed LBGs, which dominate at 1 h −1 Mpc scales. When these are taken into account, the observed LBG correlation functions are well matched by the high amplitude of clustering, shown by higher mass (M * > 10 9 M) galaxies in the simulation. The simulated cross-correlation function shows similar neutral gas densities around galaxies to those seen in the observations. The simulated and observed Lyα z-space autocorrelation functions again agree better with each other than with the β Lyα ≈ 1.3 infall model. Our overall conclusion is that, at least in the simulation, gas and galaxy peculiar velocities are generally towards the low end of expectation. Finally, little direct evidence is seen in either simulation or observations for high transmission near galaxies due to feedback, in agreement with previous results.

The Astronomical Journal, 2007

We carried out a wide-field V, I imaging survey of the Local Group dwarf spheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2 m Subaru Telescope. The survey covered an area of 26:67 ; 26:67 arcmin 2 , far beyond the tidal radius of Leo II (8.63 0), down to the limiting magnitude of V ' 26, which is roughly 1 mag deeper than the turnoff point of the main-sequence stars of Leo II. Radial number density profiles of bright and faint red giant branch (RGB) stars were found to change their slopes at around the tidal radius, and extend beyond the tidal radius with shallower slopes. A smoothed surface brightness map of Leo II suggests the existence of a small substructure (4 ; 2:5 arcmin 2 , 270 ; 170 pc 2 in physical size) of globular cluster luminosity beyond the tidal radius. We investigated the properties of the stellar population by means of a color-magnitude diagram. The horizontal branch (HB) morphology index shows a radial gradient in which red HB stars are more concentrated than blue HB stars, which is common to many Local Group dwarf spheroidal galaxies. The color distribution of RGB stars around the mean RGB sequence shows a larger dispersion at the center than in the outskirts, indicating a mixture of stellar populations at the center and a more homogeneous population in the outskirts. Based on the age estimation using subgiant branch stars, we found that although the major star formation took place $8 Gyr ago, a considerable stellar population younger than 8 Gyr is found at the center; such a younger population is insignificant in the outskirts. The following star formation history is suggested for Leo II. Star-forming activity occurred more than $8 Gyr ago throughout the galaxy at a modest star formation rate. The star-forming region gradually shrank from the outside toward the center, and star-forming activity finally dropped to $0 by $4 Gyr ago, except for the center, where a small population younger than 4 Gyr is present.

The Astrophysical Journal, 2003

In this paper we study the observational signatures of the lensing signal produced by dark matter halos with embedded misaligned disks. This issue is of particular interest at the present time since most of the observed multiple lens systems have magnification ratios and image geometries that are not well-fit by standard mass models. The presence of substructure exterior to the lens has been invoked by several authors in the context of Cold Dark Matter (CDM) models in order to explain the anamolous magnification ratios. We emphasize that the anomalous magnification ratios may be an artifact of the simple one-component mass models currently in use; the inclusion of a misaligned disk may be able to mimic the effect of substructure. These slight spatial offsets between the dark matter halo and the disk, which are likely to occur during or as a consequence of interactions or mergers, lead to complex image configurations and non-standard magnification ratios. We investigate the effects of disk misalignment on two illustrative lenses: a spiral disk embedded within a dark matter halo, and a compact disk-like component within an elliptical galaxy. The expected fraction of galaxies with a misaligned disk is estimated to be of the order of 10%. In such cases we find that the resultant lensing geometries are unusual, with high image multiplicities. The caustic structures-both radial and tangential-are drastically modified and the magnification ratios differ compared to expectations from standard lens models. The additional parameters required to specify the relative alignment of multiple mass components in the primary lens introduce yet another source of uncertainty in the mass modeling of gravitational lens systems.

The Astronomy and Astrophysics Review, 2021

In this review, I will discuss the comparison between model results and observational data for the Milky Way, the predictive power of such models as well as their limits. Such a comparison, known as Galactic archaeology, allows us to impose constraints on stellar nucleosynthesis and timescales of formation of the various Galactic components (halo, bulge, thick disk and thin disk).

Astronomy &amp; Astrophysics, 2020

Context. M 105 (NGC 3379) is an early-type galaxy in the Leo I group. The Leo I group is the nearest group that contains all main galaxy types and can thus be used as a benchmark to study the properties of the intra-group light (IGL) in low-mass groups. Aims. We present a photometric survey of planetary nebulae (PNe) in the extended halo of the galaxy to characterise its PN populations and investigate the presence of an extended PN population associated with the intra-group light. Methods. We use PNe as discrete stellar tracers of the diffuse light around M 105. These PNe were identified on the basis of their bright [O III]5007 Å emission and the absence of a broad-band continuum using automated detection techniques. We compare the PN number density profile with the galaxy surface-brightness profile decomposed into metallicity components using published photometry of the Hubble Space Telescope in two halo fields. Results. We identify 226 PNe candidates within a limiting magnitude of...

Physical Review D, 2022

Observations of high-redshift galaxies have provided us with a rich tool to study the physics at play during the epoch of reionisation. The luminosity function (LF) of these objects is an indirect tracer of the complex processes that govern galaxy formation, including those of the first dark-matter structures. In this work, we present an extensive analysis of the UV galaxy LF at high redshifts to extract cosmological and astrophysical parameters. We provide a number of phenomenological approaches in modelling the UV LF and take into account various sources of uncertainties and systematics in our analysis, including cosmic variance, dust extinction, scattering in the halo-galaxy connection, and the Alcock-Paczyński effect. Using UV LF measurements from the Hubble Space Telescope together with external data on the matter density, we derive the large-scale matter clustering amplitude to be σ8 = 0.76 +0.12 −0.14 , after marginalising over the unknown astrophysical parameters. We find that with current data this result is only weakly sensitive to our choice of astrophysical modelling, as well as the calibration of the underlying halo mass function. As a cross check, we run our analysis pipeline with mock data from the IllustrisTNG hydrodynamical simulations and find consistent results with their input cosmology. In addition, we perform a simple forecast for future space telescopes, where an improvement of roughly 30% upon our current result is expected. Finally, we obtain constraints on astrophysical parameters and the halo-galaxy connection for the models considered here. All methods discussed in this work are implemented in the form of a versatile likelihood code, GALLUMI, which we make public. The GALLUMI code and scripts for making the plots in this paper can be found here.

Monthly Notices of the Royal Astronomical Society, 2012

Analytic estimates of the viscous timescale due to cloud-cloud collisions have been as high as thousands of Gyr. Consequently, cloud collisions are widely ignored as a source of viscosity in galactic discs. However, capturing the hydrodynamics of discs in simple analytic models is a challenge, because of both the wide dynamic range and the importance of 2D and 3D effects. To test the validity of analytic models, we present estimates for the viscous timescale that are measured from 3D smoothed particle hydrodynamics simulations of disc formation and evolution. We have deliberately removed uncertainties associated with star formation and feedback, thereby enabling us to place lower bounds on the timescale for this process. We also contrast collapse simulations with results from simulations of initially stable discs and examine the impact of numerical parameters and assumptions on our work, to constrain possible systematics in our estimates. We find that cloud-collision viscous timescales are in the range of 0.6-16 Gyr, considerably shorter than previously estimated. This large discrepancy can be understood in terms of how the efficiency of collisions is included in the analytical estimates. We find that the viscous timescale only depends weakly on the number of clouds formed, and so while the viscous timescale will increase with increasing resolution, this effect is too weak to alter our conclusions.

Monthly Notices of the Royal Astronomical Society, 2017

We present the public release of the MULTIDARK-GALAXIES: three distinct galaxy catalogues derived from one of the Planck cosmology MULTIDARK simulations (i.e. MDPL2, with a volume of (1 h −1 Gpc) 3 and mass resolution of 1.5 × 10 9 h −1 M) by applying the semi-analytic models GALACTICUS, SAG, and SAGE to it. We compare the three models and their conformity with observational data for a selection of fundamental properties of galaxies like stellar mass function, star formation rate, cold gas fractions, and metallicities-noting that they sometimes perform differently reflecting model designs and calibrations. We have further selected galaxy subsamples of the catalogues by number densities in stellar mass, cold gas mass, and star formation rate in order to study the clustering statistics of galaxies. We show that despite different treatment of orphan galaxies, i.e. galaxies that lost their dark-matter host halo due to the finite mass resolution of the N-body simulation or tidal stripping, the clustering signal is comparable, and reproduces the observations in all three models-in particular when selecting samples based upon stellar mass. Our catalogues provide a powerful tool to study galaxy formation within a volume comparable to those probed by ongoing and future photometric and redshift surveys. All model data consisting of a range of galaxy propertiesincluding broad-band SDSS magnitudes-are publicly available.

Monthly Notices of the Royal Astronomical Society, 2023

Cosmological inference with large galaxy surv e ys requires theoretical models that combine precise predictions for large-scale structure with robust and flexible galaxy formation modelling throughout a sufficiently large cosmic volume. Here, we introduce the MILLENNIUMTNG (MTNG) project which combines the hydrodynamical galaxy formation model of ILLUSTRISTNG with the large volume of the MILLENNIUM simulation. Our largest hydrodynamic simulation, co v ering (500 h −1 Mpc) 3 (740 Mpc) 3 , is complemented by a suite of dark-matter-only simulations with up to 4320 3 dark matter particles (a mass resolution of 1. 32 × 10 8 h −1 M) using the fixed-and-paired technique to reduce large-scale cosmic variance. The hydro simulation adds 4320 3 gas cells, achieving a baryonic mass resolution of 2 × 10 7 h −1 M. High time-resolution merger trees and direct light-cone outputs facilitate the construction of a new generation of semi-analytic galaxy formation models that can be calibrated against both the hydro simulation and observation, and then applied to even larger volumes-MTNG includes a flagship simulation with 1.1 trillion dark matter particles and massive neutrinos in a volume of (3000 Mpc) 3. In this introductory analysis we carry out convergence tests on basic measures of non-linear clustering such as the matter power spectrum, the halo mass function and halo clustering, and we compare simulation predictions to those from current cosmological emulators. We also use our simulations to study matter and halo statistics, such as halo bias and clustering at the baryonic acoustic oscillation scale. Finally we measure the impact of baryonic physics on the matter and halo distributions.

Monthly Notices of the Royal Astronomical Society, 2017

Emission line galaxies (ELGs) are used in several ongoing and upcoming surveys (SDSS-IV/eBOSS, DESI) as tracers of the dark matter distribution. Using a new galaxy formation model, we explore the characteristics of [O II] emitters, which dominate optical ELG selections at z 1. Model [O II] emitters at 0.5 < z < 1.5 are selected to mimic the DEEP2, VVDS, eBOSS and DESI surveys. The luminosity functions of model [O II] emitters are in reasonable agreement with observations. The selected [O II] emitters are hosted by haloes with M halo ≥ 10 10.3 h −1 M , with ∼90 per cent of them being central star-forming galaxies. The predicted mean halo occupation distributions of [O II] emitters have a shape typical of that inferred for star-forming galaxies, with the contribution from central galaxies, N [O II] cen , being far from the canonical step function. The N [O II] cen can be described as the sum of an asymmetric Gaussian for discs and a step function for spheroids, which plateau below unity. The model [O II] emitters have a clustering bias close to unity, which is below the expectations for eBOSS and DESI ELGs. At z ∼ 1, a comparison with observed g-band-selected galaxy, which is expected to be dominated by [O II] emitters, indicates that our model produces too few [O II] emitters that are satellite galaxies. This suggests the need to revise our modelling of hot gas stripping in satellite galaxies.

Astronomy &amp; Astrophysics, 2013

Context. The stellar halo provides precious information about the Galaxy in its early stages of evolution because the most metal-poor (oldest) stars in the Milky Way are found there. Aims. We study the chemical evolution and formation of the Galactic halo through the analysis of its stellar metallicity distribution function and some key elemental abundance patterns. We also test the effects of a possible Population III of zero-metal stars. Methods. Starting from the two-infall model for the Galaxy, which predicts too few low-metallicity stars, we added a gas outflow during the halo phase with a rate proportional to the star formation rate through a free parameter, λ. In addition, we considered a first generation of massive zero-metal stars in this two-infall + outflow model, adopting two different top-heavy initial mass functions and specific Population III yields. Results. The metallicity distribution function of halo stars as predicted from the two-infall + outflow model agrees well with observations when the parameter λ = 14 and the time scale for the first infall, out of which the halo formed, is not longer than 0.2 Gyr, a lower value than suggested previously. Moreover, the abundance patterns [X/Fe] vs. [Fe/H] for C, N, and α-elements O, Mg, Si, S, and Ca agree well with the observational data, as in the case of the two-infall model without outflow. If Population III stars are included, under the assumption of different initial mass functions, the overall agreement of the predicted stellar metallicity distribution function with observational data is poorer than in the case of the two-infall + outflow model without Population III. Conclusions. We conclude that it is fundamental to include both a gas infall and outflow during the halo formation to explain the observed halo metallicity distribution function in the framework of a model assuming that the stars in the inner halo formed mostly in situ. Moreover, we find that there is no satisfactory initial mass function for Population III stars that reproduces the observed halo metallicity distribution function. As a consequence, there is no need for a first generation of only massive stars to explain the evolution of the Galactic halo.

Astronomy and Astrophysics, 2006

Context. Quasars trace the most massive structures at high redshifts and their presence may influence the evolution of the massive host galaxies. Aims. We study the extended Lyα emission line regions (EELRs) around seven bright, mostly radio-quiet quasars (QSOs) at 2.7 < z < 4.5, and compare luminosities with EELRs around radio-loud QSOs reported in the literature. Methods. Using integral field spectroscopy, we analyse the morphology and kinematics of the quiescent Lyα EELRs around the QSOs. Results. We find evidence for the presence of EELRs around four radio-quiet and one radio-loud QSO. All EELRs appear asymmetric and the optically brightest QSOs also have the brightest Lyα nebulae. For the two brightest nebulae we find velocities between ∼600 km s −1 at the QSO position to ∼200 km s −1 at a distance of 3−4 from the QSO and surface flux densities up to 2-3 × 10 −16 erg cm −2 s −1 arcsec −2. The five EELRs have total Lyα luminosities which correspond to ∼0.5% of the luminosities from the QSOs broad Lyα emission lines. This fraction is an order of magnitude smaller than found for EELRs around radio-loud, steep spectrum QSOs reported in the literature. While the nebulae luminosities are correlated with the QSO Lyα luminosities, we find that nebulae luminosities are not correlated with the central QSO ionising fluxes. Conclusions. The presence of gas in the EELRs can be interpreted based on two competing scenarios: either from quasar feedback mechanisms, or from infalling matter. Apart from these two effects, the Lyα flux around radio-loud objects can be enhanced due to interactions with the radio jets. The relatively fainter nebulae around radio-quiet QSOs compared to lobe-dominated radio-loud QSOs can be ascribed to this effect, or to significant differences in the environments between the two classes.

Astronomy and Astrophysics, 2012

Context. We carry out a detailed 2-D study of the ionised gas in the local universe galaxy UGC 9837. In nearby galaxies, like the galaxy in question here, the spatial distribution of the physical properties can be studied in detail, providing benchmarks for galaxy formation models. Aims. Our aim is to derive detailed and spatially resolved physical properties of the ionised gas of UGC 9837. In addition, we derive an integrated spectrum of the galaxy and study how varying spatial coverage affects the derived integrated properties. We also study how the same properties would be seen if the galaxy was placed at a higher redshift and observed as part of one of the high-z surveys. Methods. UGC 9837 was observed using the PMAS PPAK integral-field unit. The spectra were reduced and calibrated and the stellar and ionised components separated. Using strong emission line ratios of the ionised gas, the source of ionisation, the dust extinction, the star formation rate, the electron density, and the oxygen abundance derived from a total integrated spectrum, central integrated spectrum, and individual fibre spectra are studied. Finally, the same properties are studied in a spectrum whose spatial resolution is degraded to simulate high-z observations. Results. The spatial distribution of the ionised gas properties is consistent with inside-out growing scenario of galaxies. We also find that lack of spatial coverage would bias the results derived from the integrated spectrum leading, e.g., to an underestimation of ionisation and overestimation of metallicity, if only the centre of the galaxy was covered by the spectrum. Our simulation of high-z observations shows that part of the spatial information, such as dust and SFR distribution would be lost, while shallower gradients in metallicity and ionisation strength would be detected.

Science, 1995

The Astronomical Journal, 2017

We quantify the effect of the galaxy group environment (for group masses of 10 12.5 to 10 14.0 M ) on the current star formation rate (SFR) of a pure, morphologically-selected, sample of disk-dominated (i.e. late-type spiral) galaxies with redshift ≤ 0.13. The sample embraces a full representation of quiescent and star-forming disks with stellar mass M * ≥ 10 9.5 M . We focus on the effects on SFR of interactions between grouped galaxies and the putative intra-halo medium (IHM) of their host group dark matter halos, isolating these effects from those induced through galaxy-galaxy interactions, and utilising a radiation transfer analysis to remove the inclination-dependence of derived SFRs. The dependence of SFR on M * is controlled for by measuring offsets ∆log(ψ * ) of grouped galaxies about a single power-law relation in specific SFR, ψ * ∝ M -0.45±0.01 * , exhibited by non-grouped "field" galaxies in the sample. While a small minority of the group satellites are strongly quenched, the group centrals, and the large majority of satellites, exhibit levels of ψ * statistically indistinguishable from their field counterparts, for all M * , albeit with a higher scatter of 0.44 dex about the field reference relation (vs. 0.27 dex for the field). Modelling the distributions in ∆log(ψ * ), we find that: (i) after infall into groups, disk-dominated galaxies continue to be characterized by a similar rapid cycling of gas into and out of their ISM shown prior to infall, with inflows and outflows of respectively ∼ 1.5 -5 x SFR and ∼ 1 -4 x SFR, and (ii) that the independence of the continuity of these gas flow cycles on M * appears inconsistent with the required fuelling being sourced from gas in the circum-galactic medium on scales of ∼ 100 kpc. Instead, our data favor on-going fuelling of satellites from the IHM of the host group halo on ∼ Mpc scales, i.e. from gas not initially associated with the galaxies upon infall. Consequently, the color-density relation of the galaxy population as a whole would appear to be primarily due to a change in the mix of disk-and spheroid-dominated morphologies in the denser group environment compared to the field, rather than to a reduced propensity of the IHM in higher mass structures to cool and accrete onto galaxies. We also suggest that the required substantial accretion of IHM gas by satellite disk-dominated galaxies will lead to a progressive reduction in the specific angular momentum of these systems, thereby representing an efficient secular mechanism to transform morphology from star-forming disk-dominated types to more passive spheroid-dominated types.

Monthly Notices of the Royal Astronomical Society, 2021

In this work, we combine size and stellar mass measurements from the Sloan Digital Sky Server (SDSS) with the group finder algorithm of Rodriguez & Merchán in order to determine the stellar and halo mass-size relations of central and satellite galaxies separately. We show that, while central and satellite galaxies display similar stellar mass-size relations, their halo mass-size relations differ significantly. As expected, more massive haloes tend to host larger central galaxies. However, the size of satellite galaxies depends only slightly on halo virial mass. We show that these results are compatible with a remarkably simple model in which the size of central and satellite galaxies scales as the cubic root of their host halo mass, with the normalization for satellites being ∼ 30 % smaller than that for central galaxies, which can be attributed to tidal stripping. We further check that our measurements are in excellent agreement with predictions from the IllustrisTNG hydrodynamical simulation. In the second part of this paper, we analyse how the clustering properties of central and satellite galaxies depend on their size. We demonstrate that, independently of the stellar mass threshold adopted, smaller galaxies are more tightly clustered than larger galaxies when either the entire sample or only satellites are considered. The opposite trend is observed on large scales when the size split is performed for the central galaxies alone. Our results place significant constraints for halo-galaxy connection models that link galaxy size with the properties of their hosting haloes.

Monthly Notices of the Royal Astronomical Society, 2020

Photoelectric heating (PEH) influences the temperature and density of the interstellar medium (ISM), potentially also affecting star formation. PEH is expected to have a stronger effect on massive galaxies, as they host larger dust reservoirs compared to dwarf systems. Accordingly, in this paper, we study PEH effects in Milky Way-like galaxies using a smoothed particle hydrodynamics code, which self-consistently implements the evolution of the gas, dust, and interstellar radiation field. Dust evolution includes dust formation by stars, destruction by SNe, and growth in dense media. We find that PEH suppresses star formation due to the excess heating that reduces the ISM density. This suppression is seen across the entire range of gas fractions, star-formation recipes, dust models, and PEH efficiencies investigated by our code. The suppression ranges from negligible values to approximately a factor of five depending on the specific implementation. Galaxy models having higher gas frac...

The Astrophysical Journal, 2017

Local extremely metal-poor galaxies (XMPs) are of particular astrophysical interest since they allow us to look into physical processes characteristic of the early universe, from the assembly of galaxy disks to the formation of stars in conditions of low metallicity. Given the luminosity-metallicity relationship, all galaxies fainter than M r ;-13 are expected to be XMPs. Therefore, XMPs should be common in galaxy surveys. However, they are not common, because several observational biases hamper their detection. This work compares the number of faint XMPs in the SDSS-DR7 spectroscopic survey with the expected number, given the known biases and the observed galaxy luminosity function (LF). The faint end of the LF is poorly constrained observationally, but it determines the expected number of XMPs. Surprisingly, the number of observed faint XMPs (∼10) is overpredicted by our calculation, unless the upturn in the faint end of the LF is not present in the model. The lack of an upturn can be naturally understood if most XMPs are central galaxies in their low-mass dark matter halos, which are highly depleted in baryons due to interaction with the cosmic ultraviolet background and to other physical processes. Our result also suggests that the upturn toward low luminosity of the observed galaxy LF is due to satellite galaxies.

International Journal of Modern Physics D, 2015

In this review we consider in detail different theoretical topics associated with interaction in the dark sector. We study linear and nonlinear interactions which depend on the dark matter and dark energy densities. We consider a number of different models (including the holographic dark energy and dark energy in a fractal universe), with interacting dark energy and dark matter, have done a thorough analysis of these models. The main task of this review was not only to give an idea about the modern set of different models of dark energy, but to show how much can be diverse dynamics of the universe in these models. We find that the dynamics of a universe that contains interaction in the dark sector can differ significantly from the Standard Cosmological Model.

Astronomy & Astrophysics, 2021

Aims. We study the dependence of the dynamical properties of massive (M ≥ 1.5 × 1013 M⊙ h−1) dark matter halos on their environment in a whole-sky ΛCDM light-cone simulation extending to z ∼ 0.65. The properties of interest for this study are the halo shape (parametrized via its principal axes), spin and virialization status, the alignment of halo spin and shape, as well as the shape-shape and spin-spin alignments among halo neighbors. Methods. We define the halo environment using the notion of halo isolation status determined by the distance to its nearest neighbor. This defines a maximum spherical region around each halo devoid of other halos, above the catalog threshold mass. We consider “close halo pairs” to be pairs separated by a distance that is lower than a specific threshold. In order to decontaminate our results from the known dependence of halo dynamical properties on mass, we used a random sampling procedure to compare properties of similar halo abundance distributions. ...

Astronomy &amp; Astrophysics, 2019

We present a synthetic galaxy lightcone specially designed for narrow-band optical photometric surveys. To reduce time-discreteness effects, unlike previous works, we directly include the lightcone construction in the L-Galaxies semi-analytic model applied to the subhalo merger trees of the Millennium simulation. Additionally, we add a model for the nebular emission in star-forming regions, which is crucial for correctly predicting the narrow- and medium-band photometry of galaxies. Specifically, we consider, individually for each galaxy, the contribution of 9 different lines: Lyα (1216 Å), Hβ (4861 Å), Hα (6563 Å), [O II] (3727 Å, 3729 Å), [O III] (4959 Å, 5007 Å), [Ne III] (3870 Å), [O I] (6300 Å), [N II] (6548 Å, 6583 Å), and [S II] (6717 Å, 6731 Å). We validate our lightcone by comparing galaxy number counts, angular clustering, and Hα, Hβ, [O II], and [O III]5007 luminosity functions to a compilation of observations. As an application of our mock lightcones, we generated catalo...