Monthly Notices of the Royal Astronomical Society, 2014
The cosmic web is the largest scale manifestation of the anisotropic gravitational collapse of ma... more The cosmic web is the largest scale manifestation of the anisotropic gravitational collapse of matter. It represents the transitional stage between linear and non-linear structures and contains easily accessible information about the early phases of structure formation processes. Here we investigate the characteristics and the time evolution of morphological components. Our analysis involves the application of the NEXUS Multiscale Morphology Filter (MMF) technique, predominantly its NEXUS+ version, to high resolution and large volume cosmological simulations. We quantify the cosmic web components in terms of their mass and volume content, their density distribution and halo populations. We employ new analysis techniques to determine the spatial extent of filaments and sheets, like their total length and local width. This analysis identifies clusters and filaments as the most prominent components of the web. In contrast, while voids and sheets take most of the volume, they correspond to underdense environments and are devoid of group-sized and more massive haloes. At early times the cosmos is dominated by tenuous filaments and sheets, which, during subsequent evolution, merge together, such that the present day web is dominated by fewer, but much more massive, structures. The analysis of the mass transport between environments clearly shows how matter flows from voids into walls, and then via filaments into cluster regions, which form the nodes of the cosmic web. We also study the properties of individual filamentary branches, to find long, almost straight, filaments extending to distances larger than 100 h −1 Mpc. These constitute the bridges between massive clusters, which seem to form along approximatively straight lines.
Journal of Cosmology and Astroparticle Physics, 2015
We study lensing by voids in Cubic Galileon and Nonlocal gravity cosmologies, which are examples ... more We study lensing by voids in Cubic Galileon and Nonlocal gravity cosmologies, which are examples of theories of gravity that modify the lensing potential. We find voids in the dark matter and halo density fields of N-body simulations and compute their lensing signal analytically from the void density profiles, which we show are well fit by a simple analytical formula. In the Cubic Galileon model, the modifications to gravity inside voids are not screened and they approximately double the size of the lensing effects compared to GR. The difference is largely determined by the direct effects of the fifth force on lensing and less so by the modified density profiles. For this model, we also discuss the subtle impact on the force and lensing calculations caused by the screening effects of haloes that exist in and around voids. In the Nonlocal model, the impact of the modified density profiles and the direct modifications to lensing are comparable, but they boost the lensing signal by only ≈ 10%, compared with that of GR. Overall, our results suggest that lensing by voids is a promising tool to test models of gravity that modify lensing.
In this thesis, the classical field model developed by Krasnitz et al. is used to compute quark a... more In this thesis, the classical field model developed by Krasnitz et al. is used to compute quark and photon production in heavy ion collisions. The first part of the thesis serves as an independent verification of previous results for quark production. To do so, an iterative method is developed to solve the non-linear system of equations that gives the initial condition for the gluonic field. In the second part, the expression giving the photon production rate is simplified using the symmetries and properties of the Color Glass Condensate and McLerran-Venugopalan models. From the two Feynman diagrams that give the leading order contribution, one is much larger than the other. The dominant diagram is given by a continuum spectrum with a very prominent peak superimposed on it. Dans cette thèse, le modèle développé par Krasnitz et al. basé sur les champs classiques est utilisé pour calculer la production de quarks et de photons dans les collisions d'ions lourds. La première partie d...
We study the formation and evolution of the cosmic web, using the high-resolution CosmoGrid \$\La... more We study the formation and evolution of the cosmic web, using the high-resolution CosmoGrid \$\Lambda\$CDM simulation. In particular, we investigate the evolution of the large-scale structure around void halo groups, and compare this to observations of the VGS-31 galaxy group, which consists of three interacting galaxies inside a large void. The structure around such haloes shows a great deal of tenuous structure, with most of such systems being embedded in intra-void filaments and walls. We use the Nexus+ algorithm to detect walls and filaments in CosmoGrid, and find them to be present and detectable at every scale. The void regions embed tenuous walls, which in turn embed tenuous filaments. We hypothesize that the void galaxy group of VGS-31 formed in such an environment.
Monthly Notices of the Royal Astronomical Society, 2015
The detection of planar structures within the satellite systems of both the Milky Way (MW) and An... more The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model (ΛCDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in ΛCDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high resolution cosmological simulations. We find that planar structures are very common, and that ∼10% of ΛCDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and ΛCDM predictions. In fact, ∼10% of ΛCDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively.
Monthly Notices of the Royal Astronomical Society, 2015
We investigate the angular and kinematic distributions of satellite galaxies around a large sampl... more We investigate the angular and kinematic distributions of satellite galaxies around a large sample of bright isolated primaries in the spectroscopic and photometric catalogues of the Sloan Digital Sky Survey (SDSS). We detect significant anisotropy in the spatial distribution of satellites. To test whether this anisotropy could be related to the rotating disks of satellites recently found by Ibata et al. in a sample of SDSS galaxies, we repeat and extend their analysis. Ibata et al. found an excess of satellites on opposite sides of their primaries having anticorrelated radial velocities. We find that this excess is sensitive to small changes in the sample selection criteria which can greatly reduce its significance. In addition, we find no evidence for correspondingly correlated velocities for satellites observed on the same side of their primaries, which would be expected for rotating disks of satellites. We conclude that the detection of coherent rotation in the satellite population in current observational samples is not robust. We compare our data to the ΛCDM Millennium simulations populated with galaxies according to the semi-analytic model of Guo et al. We find excellent agreement with the spatial distribution of satellites in the SDSS data and the lack of a strong signal from coherent rotation.
We investigate the characteristics and the time evolution of the cosmic web from redshift, z=2, t... more We investigate the characteristics and the time evolution of the cosmic web from redshift, z=2, to present time, within the framework of the NEXUS+ algorithm. This necessitates the introduction of new analysis tools optimally suited to describe the very intricate and hierarchical pattern that is the cosmic web. In particular, we characterize filaments (walls) in terms of their linear (surface) mass density. This is very good in capturing the evolution of these structures. At early times the cosmos is dominated by tenuous filaments and sheets, which, during subsequent evolution, merge together, such that the present day web is dominated by fewer, but much more massive, structures. We also show that voids are more naturally described in terms of their boundaries and not their centres. We illustrate this for void density profiles, which, when expressed as a function of the distance from void boundary, show a universal profile in good qualitative agreement with the theoretical shell-cro...
Monthly Notices of the Royal Astronomical Society, 2014
We use the distribution of maximum circular velocities, V max , of satellites in the Milky Way (M... more We use the distribution of maximum circular velocities, V max , of satellites in the Milky Way (MW) to constrain the virial mass, M 200 , of the Galactic halo under an assumed prior of a ΛCDM universe. This is done by analysing the subhalo populations of a large sample of halos found in the Millennium II cosmological simulation. The observation that the MW has at most three subhalos with V max 30 km/s requires a halo mass M 200 1.4 × 10 12 M , while the existence of the Magellanic Clouds (assumed to have V max 60 km/s) requires M 200 1.0 × 10 12 M . The first of these conditions is necessary to avoid the "too-big-to-fail" problem highlighted by Boylan-Kolchin et al., while the second stems from the observation that massive satellites like the Magellanic Clouds are rare. When combining both requirements, we find that the MW halo mass must lie in the range 0.25 M 200 /(10 12 M ) 1.4 at 90% confidence. The gap in the abundance of Galactic satellites between 30 km/s V max 60 km/s places our galaxy in the tail of the expected satellite distribution.
One of the important unknowns of current cosmology concerns the effects of the large scale distri... more One of the important unknowns of current cosmology concerns the effects of the large scale distribution of matter on the formation and evolution of dark matter haloes and galaxies. One main difficulty in answering this question lies in the absence of a robust and natural way of identifying the large scale environments and their characteristics. This work summarizes the NEXUS+ formalism which extends and improves our multiscale scale-space MMF method. The new algorithm is very successful in tracing the Cosmic Web components, mainly due to its novel filtering of the density in logarithmic space. The method, due to its multiscale and hierarchical character, has the advantage of detecting all the cosmic structures, either prominent or tenuous, without preference for a certain size or shape. The resulting filamentary and wall networks can easily be characterized by their direction, thickness, mass density and density profile. These additional environmental properties allows to us to investigate not only the effect of environment on haloes, but also how it correlates with the environment characteristics.
Monthly Notices of the Royal Astronomical Society, 2014
We study the substructure population of Milky Way (MW)-mass halos in the ΛCDM cosmology using a n... more We study the substructure population of Milky Way (MW)-mass halos in the ΛCDM cosmology using a novel procedure to extrapolate subhalo number statistics beyond the resolution limit of N-body simulations. The technique recovers the mean and the variance of the subhalo abundance, but not its spatial distribution. It extends the dynamic range over which precise statistical predictions can be made by the equivalent of performing a simulation with 50 times higher resolution, at no additional computational cost. We apply this technique to MW-mass halos, but it can easily be applied to halos of any mass. We find up to 20% more substructures in MW-mass halos than found in previous studies. Our analysis lowers the mass of the MW halo required to accommodate the observation that the MW has only three satellites with a maximum circular velocity V max 30 km/s in the ΛCDM cosmology. The probability of having a subhalo population similar to that in the MW is 20% for a virial mass, M 200 = 1 × 10 12 M and practically zero for halos more massive than M 200 = 2 × 10 12 M .
Monthly Notices of the Royal Astronomical Society, 2013
We introduce the NEXUS algorithm for the identification of Cosmic Web environments: clusters, fil... more We introduce the NEXUS algorithm for the identification of Cosmic Web environments: clusters, filaments, walls and voids. This is a multiscale and automatic morphological analysis tool that identifies all the cosmic structures in a scale free way, without preference for a certain size or shape. We develop the NEXUS method to incorporate the density, tidal field, velocity divergence and velocity shear as tracers of the Cosmic Web. We also present the NEXUS+ procedure which, taking advantage of a novel filtering of the density in logarithmic space, is very successful at identifying the filament and wall environments in a robust and natural way.
Monthly Notices of the Royal Astronomical Society, 2013
We study the formation and evolution of filamentary configurations of dark matter haloes in voids... more We study the formation and evolution of filamentary configurations of dark matter haloes in voids. Our investigation uses the high-resolution ΛCDM simulation Cos-moGrid to look for void systems resembling the VGS 31 elongated system of three interacting galaxies that was recently discovered by the Void Galaxy Survey (VGS) inside a large void in the SDSS galaxy redshift survey. HI data revealed these galaxies to be embedded in a common elongated envelope, possibly embedded in intravoid filament.
Journal of Cosmology and Astroparticle Physics, 2013
We investigate the effect of hypothetical long-range scalar interactions present in the Dark Matt... more We investigate the effect of hypothetical long-range scalar interactions present in the Dark Matter sector on to the internal properties of Dark Matter haloes. We employ a particular formulation of such modified cosmology called the ReBEL model, in which the additional interactions between Dark Matter particles are modeled on a phenomenological level as modification of the gravity. We perform a series of high resolution cosmological N-body simulations of 512 3 particles for five different ReBEL models and for one reference ΛCDM model. We compare density profiles, velocity curves, the spin and shape parameters (triaxiality and ellipcities) between models using statistical distributions of these quantities as well as direct comparison of haloes cross-correlated between the ReBEL and the ΛCDM models. We also test and compare the state of the virialisation of the haloes in our models. We find that statistically the ReBEL halo density profiles are also described quite well by the Navarro-Frenk-White profile, but we denote an incerase of the mean virial concentration parameter by 6% to 15% for our range of tested ReBEL models. We also find that the velocity curves for our modified gravity haloes reach their maxima at smaller radii when compared to standard cosmology haloes. We also denote a slight increase of the mean of the halo spin parameter reflecting a bit higher rotational support of the haloes due to the scalar interacting Dark Matter. On the other hand we do not find any significant differences in shapes of the haloes between standard and modified gravity paradigms. Finally we report that in the class of modified Dark Matter models we study here the overall virialisation state of DM halo is higher compared to the ΛCDM haloes. The cosmological implications and prospects for using haloes as probes of modified gravity is briefly discussed.
Monthly Notices of the Royal Astronomical Society, 2016
We introduce Copernicus Complexio (COCO), a high-resolution cosmological N-body simulation of str... more We introduce Copernicus Complexio (COCO), a high-resolution cosmological N-body simulation of structure formation in the ΛCDM model. COCO follows an approximately spherical region of radius ∼ 17.4h −1 Mpc, in which the particle mass is 1.1 × 10 5 h −1 M , embedded in a much larger periodic cube followed at lower resolution. Thus, the resolution in the inner volume is 60 times better than in the Millennium-II simulation. COCO gives the dark matter halo mass function over eight orders of magnitude in halo mass; it forms ∼ 60 halos of galactic size, each resolved with about 10 million particles. The concentration-mass relation of COCO halos deviates from a single power law for masses M 200 < a few × 10 8 h −1 M , where it flattens in agreement with results by Sanchez-Conde et al. We confirm the power-law character of the subhalo mass function, N (> µ) ∝ µ −s , down to a reduced subhalo mass M sub /M 200 ≡ µ = 10 −6 , with a best-fit power-law index, s = 0.94, for hosts of mass M 200 = 10 12 h −1 M , increasing very slowly with host mass. The host-mass invariance of the reduced maximum circular velocity function of subhalos, ν ≡ V max /V 200 , hinted at in previous simulations, is clearly demonstrated over five orders of magnitude in host mass. Similarly, we find that the average, normalised radial distribution of subhalos is approximately universal (i.e. independent of subhalo mass), as previously suggested by the AQUARIUS simulations of individual halos. Finally, we find that at fixed physical subhalo size, subhaloes in lower mass hosts typically have lower central densities than those in higher mass hosts.
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Papers by Marius Cautun