Amina Helmi

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Papers by Amina Helmi

Detecting Halo Streams with GAIA

We investigate what the proposed ESA astrometric satellite GAIA will reveal by observing the halo... more We investigate what the proposed ESA astrometric satellite GAIA will reveal by observing the halo of the Milky Way. Specifically, we look for halo streams which are the signatures left by the merging/accretion events experienced by a typical galaxy like the Milky Way. We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to generate artificial GAIA halo catalogs. We recover the streams by searching for peaks in angular momentum space.

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The Aquarius Project: the subhalos of galactic halos

by Amina Helmi and Mark Vogelsberger

We have performed the largest ever particle simulation of a Milky Way-sized dark matter halo, and... more We have performed the largest ever particle simulation of a Milky Way-sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far. We have also simulated a sample of 6 ultrahighly resolved Milky-way sized halos, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300,000 gravitationally bound subhalos within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors up to 1800 accurately reproduce the largest subhalos with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subhalos within subhalos, but contrary to recent claims, we find less substructure in subhalos than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3% within 100 kpc, and to be below 0.1% within the Solar Circle. The inner density profiles of subhalos show no sign of converging to a fixed asymptotic slope and are well fit by gently curving profiles of Einasto form. The mean concentrations of isolated halos are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km s -1 , an extrapolation over some 5 orders of magnitude in mass. However, at equal maximum circular velocity, subhalos are more concentrated than field halos, with a characteristic density that is typically ∼ 2.6 times larger and increases with decreasing distance from halo centre.

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The Formation of the Outer Halo and the Mass of the Galaxy

With distant halo stars we can now map the outer Galactic halo, where previously only satellite g... more With distant halo stars we can now map the outer Galactic halo, where previously only satellite galaxies and a few globular clusters were known. This is crucial for an understanding of the manner and timescale of the assembly of the Milky Way's stellar halo and the total mass and shape of its dark halo. Our previous work has proved that the Washington photometric system, in conjunction with spectroscopy, is capable of efficiently identifying halo giants out to 100 kpc or more. We request followup spectroscopy to map velocity substructure, more likely visible in the outer halo because of the long dynamical timescales (our early results give tantalizing suggestions of streams), and to utilize metal-poor giants to constrain the dark halo.

Matching the dark matter profiles of dSph galaxies with those of simulated satellites: a two parameter comparison

by Carlos Vera-ciro and Amina Helmi

We compare the dark matter halos' structural parameters derived for four Milky Way dwarf sphe... more We compare the dark matter halos' structural parameters derived for four Milky Way dwarf spheroidal galaxies to those of subhalos found in cosmological $N$-body simulations. We confirm that estimates of the mass at a single fixed radius are fully consistent with the observations. However, when a second structural parameter such as the logarithmic slope of the dark halo density profile measured close to the half-light radius is included in the comparison, we find little to no overlap between the satellites and the subhalos. Typically the right mass subhalos have steeper profiles at these radii than measurements of the dSph suggest. Using energy arguments we explore if it is possible to solve this discrepancy by invoking baryonic effects. Assuming that feedback from supernovae can lead to a reshaping of the halos, we compute the required efficiency and find entirely plausible values for a significant fraction of the subhalos and even as low as 0.1%. This implies that care must be ...

What can Gaia proper motions tell us about Milky Way dwarf galaxies?

EAS Publications Series, 2014

We present a proper-motion study on models of the dwarf spheroidal galaxy Sculptor, based on the ... more We present a proper-motion study on models of the dwarf spheroidal galaxy Sculptor, based on the predicted proper-motion accuracy of Gaia measurements. Gaia will measure proper motions of several hundreds of stars for a Sculptor-like system. Even with an uncertainty on the proper motion of order 1.5 times the size of an individual proper-motion value of ∼ 10 mas/century, we find that it is possible to recover Sculptor's systemic proper motion at its distance of 79 kpc.

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COMPLEXITY ON DWARF GALAXY SCALES: A BIMODAL DISTRIBUTION FUNCTION IN SCULPTOR

The Astrophysical Journal, 2014

ABSTRACT In previous work we have presented Schwarzschild models of the Sculptor dSph, demonstrat... more ABSTRACT In previous work we have presented Schwarzschild models of the Sculptor dSph, demonstrating that this system could be embedded in dark matter halos that are either cusped or cored. Here we show that the non-parametric distribution function recovered through Schwarschild&#39;s method is bimodal in energy and angular momentum space for all best fitting mass models explored. We demonstrate that this bimodality is directly related to the two components known to be present in Sculptor through stellar populations analysis, although our method is purely dynamical in nature and does not use this prior information. It therefore constitutes independent confirmation of the existence of two physically distinct dynamical components in Sculptor and suggests a rather complex assembly history for this dwarf galaxy.

Mapping the Galactic Halo VIII: Quantifying substructure

The Astrophysical Journal

We have measured the amount of kinematic substructure in the Galactic halo using the final data s... more We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the star...

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THE IMPRINTS OF THE GALACTIC BAR ON THE THICK DISK WITH RAVE

The Astrophysical Journal, 2015

We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius ce... more We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius centered on the Sun, in the RAVE dataset. We find clear asymmetries in the v R -v φ velocity distributions of thin and thick disk stars: there are more stars moving radially outwards for low azimuthal velocities and more radially inwards for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick disk stars should no longer be described by Schwarzschild's, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully.

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The Galactic bar and the large scale velocity gradients in the Galactic disk

Astronomy & Astrophysics, 2014

Aims. We investigate whether the cylindrical (galactocentric) radial velocity gradient of ∼ −3 km... more Aims. We investigate whether the cylindrical (galactocentric) radial velocity gradient of ∼ −3 km s −1 kpc −1 , directed radially from the Galactic center and recently observed in the stars of the Solar Neighborhood with the RAVE survey, can be explained by the resonant effects of the bar near the Solar Neighborhood. Methods. We compare the results of test particle simulations of the Milky Way with a potential including a rotating bar with observations from the RAVE survey. To this end we apply the RAVE selection function to the simulations, and convolve these with the characteristic RAVE errors. We explore different "solar neighborhoods" in the simulations as well as different bar models. Results. We find that the bar induces a negative radial velocity gradient at every height from the Galactic plane, outside the Outer Lindblad Resonance, and for angles from the long axis of the bar compatible with the current estimates. The selection function and errors do not wash away the gradient, but often make it steeper, especially near the Galactic plane because this is where the RAVE survey is less radially extended. No gradient in the vertical velocity is present in our simulations, from which we may conclude that this cannot be induced by the bar.

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Elemental abundances in Milky Way-like galaxies from a hierarchical galaxy formation model

We develop a new method to account for the finite lifetimes of stars and trace individual abundan... more We develop a new method to account for the finite lifetimes of stars and trace individual abundances within a semi-analytic model of galaxy formation. At variance with previous methods, based on the storage of the (binned) past star formation history of model galaxies, our method projects the information about the metals produced by each simple stellar population (SSP) in the future. Using this approach, an accurate accounting of the timings and properties of the individual SSPs composing model galaxies is possible. We analyse the dependence of our chemical model on various ingredients, and apply it to six simulated haloes of roughly Milky Way mass and with no massive close neighbour at z=0. For all models considered, the [Fe/H] distributions of the stars in the disc component are in good agreement with Milky Way data, while for the spheroid component (whose formation we model only through mergers) these are offset low with respect to observational measurements for the Milky Way bulge. This is a consequence of narrow star formation histories, with relatively low rates of star formation. The slow recycling of gas and energy from supernovae in our chemical model has important consequences on the predicted star formation rates, which are systematically lower than the corresponding rates in the same physical model but with an instantaneous recycling approximation. The halo that resembles most our Galaxy in terms of its global properties also reproduces the observed relation between the average metallicity and luminosity of the Milky Way satellites, albeit with a slightly steeper slope.

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Binary white dwarfs in the halo of the Milky Way

Astronomy & Astrophysics, 2014

ABSTRACT Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in orde... more ABSTRACT Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation $10-13$ Gyrs ago. Gaia will be the first instrument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities ($\log g &lt; 6$) and reach luminosities $\log(L/L_\odot) &gt; 0$ in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries.

The Aquarius Project: Cold Dark Matter under a Numerical Microscope

High Performance Computing in Science and Engineering, Garching/Munich 2007, 2009

ABSTRACT The ‘Aquarius’ project currently performs the first ever one-billion particle simulation... more ABSTRACT The ‘Aquarius’ project currently performs the first ever one-billion particle simulation of a Milky Way-sized dark matter halo, improving resolution by a factor of more than 15 relative to previously published simulations of this type. This enables dramatic advances in our understanding of the structure and substructure of dark matter in our Galaxy. Our project seeks clues to the nature of the dark matter and aims to advance strategies for exploring the formation of our Galaxy, for searching for signals from dark matter annihilation, and for designing experiments for direct detection of dark matter. Here we report on the status of our calculations carried out on the HLRB-2 thus far, and discuss some of the early results we obtained. Our results show much better convergence for the properties of dark matter substructures than ever reported in the literature before. For the first time, we can reliably probe the central dark matter density cusp into a regime where the local logarithmic slope becomes shallower than−1. We also provide a description of the simulation code GADGET-3 developed specifically for this project, and highlight the new parallelization techniques we employed to deal with the extremely tightly coupled nature and high dynamic range of our simulations.

Binary White Dwarfs in the Galactic Halo

Proceedings of the International Astronomical Union, 2013

ABSTRACT We use the stellar population synthesis code SeBa (Portegies Zwart &amp; Verbunt (19... more ABSTRACT We use the stellar population synthesis code SeBa (Portegies Zwart &amp; Verbunt (1996), Toonen, Nelemans &amp; Portegies Zwart (2012)) to study the halo white dwarf population. Here we assume a Kroupa initial mass function and compare 4 models, varying two parameters: the star formation (SF) history of the halo (either continuous SF during 2.5 Gyr, which started 13.2 Gyr ago, or a SF burst during 360 Myr, which started 12.9 Gyr ago - see the left panel of the figure) and the binary fraction of the halo (either 100% single stars, or 100% binaries). White dwarf cooling models (Althaus et al. (2009) and Renedo et al. (2010)) allow us to plot the halo white dwarf luminosity function for these 4 models, as is done in the right panel of the figure. Combined with an assumption about the density distribution of halo stars, we will study which of these white dwarfs Gaia can see, and what that can tell us about the initial parameter distributions in the halo. In the near future, we plan to use the Munich-Groningen semi-analytical galaxy formation model (Starkenburg et al. (2013)), to obtain key ingredients for the population synthesis modeling, such as a realistic star formation history (see the left panel of the figure).

Interstellar medium. Pseudo-three-dimensional maps of the diffuse interstellar band at 862 nm

Science (New York, N.Y.), Jan 15, 2014

The diffuse interstellar bands (DIBs) are absorption lines observed in visual and near-infrared s... more The diffuse interstellar bands (DIBs) are absorption lines observed in visual and near-infrared spectra of stars. Understanding their origin in the interstellar medium is one of the oldest problems in astronomical spectroscopy, as DIBs have been known since 1922. In a completely new approach to understanding DIBs, we combined information from nearly 500,000 stellar spectra obtained by the massive spectroscopic survey RAVE (Radial Velocity Experiment) to produce the first pseudo-three-dimensional map of the strength of the DIB at 8620 angstroms covering the nearest 3 kiloparsecs from the Sun, and show that it follows our independently constructed spatial distribution of extinction by interstellar dust along the Galactic plane. Despite having a similar distribution in the Galactic plane, the DIB 8620 carrier has a significantly larger vertical scale height than the dust. Even if one DIB may not represent the general DIB population, our observations outline the future direction of DIB ...

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Constraining the Milky Way potential using the dynamical kinematic substructures

by Amina Helmi and G. Monari

EPJ Web of Conferences, 2012

We present a method to constrain the potential of the non-axisymmetric components of the Galaxy u... more We present a method to constrain the potential of the non-axisymmetric components of the Galaxy using the kinematics of stars in the solar neighborhood. The basic premise is that dynamical substructures in phase-space (i.e. due to the bar and/or spiral arms) are associated with families of periodic or irregular orbits, which may be easily identified in orbital frequency space. We use the "observed" positions and velocities of stars as initial conditions for orbital integrations in a variety of gravitational potentials. We then compute their characteristic frequencies, and study the structure present in the frequency maps. We find that the distribution of dynamical substructures in velocity-and frequency-space is best preserved when the integrations are performed in the "true" gravitational potential.

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RAVE DR2 distance catalogue (Breddels+, 2010)

by James Binney, Amina Helmi, and Maarten Breddels

We used stellar models combined with atmospheric properties from RAVE (effective temperature, sur... more We used stellar models combined with atmospheric properties from RAVE (effective temperature, surface gravity and metallicity) and (J-Ks) photometry from archival sources to derive absolute magnitudes. In combination with apparent magnitudes, sky coordinates, proper motions from a variety of sources and radial velocities from RAVE, we are able to derive the full 6D phase-space coordinates for a large sample of

4MOST: 4-metre multi-object spectroscopic telescope

Ground-based and Airborne Instrumentation for Astronomy IV, 2012

The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim ... more The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on the telescope to perform a 5 year public survey yielding more than 20 million spectra at resolution R∼5000 (λ=390-1000 nm) and more than 2 million spectra at R~20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid. Initial design and performance estimates for the wide-field corrector concepts are presented. Two fibre positioner concepts are being considered for 4MOST. The first one is a Phi-Theta system similar to ones used on existing and planned facilities. The second one is a new R-Theta concept with large patrol area. Both positioner concepts effectively address the issues of fibre focus and pupil pointing. The 4MOST spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high-and low-resolution fibres. A full facility simulator is being developed to guide trade-off decisions regarding the optimal field-of-view, number of fibres needed, and the relative fraction of high-to-low resolution fibres. The simulator takes mock catalogues with template spectra from Design Reference Surveys as starting point, calculates the output spectra based on a throughput simulator, assigns targets to fibres based on the capabilities of the fibre positioner designs, and calculates the required survey time by tiling the fields on the sky. The 4MOST consortium aims to deliver the full 4MOST facility by the end of 2018 and start delivering high-level data products for both consortium and ESO community targets a year later with yearly increments.

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APASS LANDOLT-SLOAN BVgri PHOTOMETRY OF RAVE STARS. I. DATA, EFFECTIVE TEMPERATURES, AND REDDENINGS

The Astronomical Journal, 2014

We provide APASS photometry in the Landolt BV and Sloan g ′ r ′ i ′ bands for all the 425,743 sta... more We provide APASS photometry in the Landolt BV and Sloan g ′ r ′ i ′ bands for all the 425,743 stars included in the latest 4th RAVE Data Release. The internal accuracy of the APASS photometry of RAVE stars, expressed as error of the mean of data obtained and separately calibrated over a median of 4 distinct observing epochs and distributed between 2009 and 2013, is 0.013, 0.012, 0.012, 0.014 and 0.021 mag for B,V ,g ′ ,r ′ and i ′ band, respectively. The equally high external accuracy of APASS photometry has been verified on secondary Landolt and Sloan photometric standard stars not involved in the APASS calibration process, and on a large body of literature data on field and cluster stars, confirming the absence of offsets and trends. Compared with the Carlsberg Meridian Catalog (CMC-15), APASS astrometry of RAVE stars is accurate to a median value of 0.098 arcsec. Brightness distribution functions for the RAVE stars have been derived in all bands. APASS photometry of RAVE stars, augmented by 2MASS JHK infrared data, has been χ 2 fitted to a densely populated synthetic photometric library designed to widely explore in temperature, surface gravity, metallicity and reddening. Resulting T eff and E B−V , computed over a range of options, are provided and discussed, and will be kept updated in response to future APASS and RAVE data releases. In the process it is found that the reddening caused by an homogeneous slab of dust, extending for 140 pc on either side of the Galactic plane and responsible for E poles B−V =0.036±0.002 at the galactic poles, is a suitable approximation of the actual reddening encountered at Galactic latitudes |b|≥25 • .

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On the Nature of the Ringlike Structure in the Outer Galactic Disk

The Astrophysical Journal, 2003

We examine the tidal disruption of satellite galaxies in a cosmological simulation of the formati... more We examine the tidal disruption of satellite galaxies in a cosmological simulation of the formation of a disk galaxy in the ΛCDM scenario. We find that the disruption of satellite galaxies in orbits roughly coplanar with the disk leads naturally to the formation of ring-like stellar structures similar to that recently discovered in the outer disk of the Milky Way. Two interpretations appear plausible in this context. One is that the ring is a transitory, localized radial density enhancement reflecting the apocenter of particles stripped from a satellite during a recent pericentric passage (a "tidal arc" reminiscent of the tidal arms seen in disk galaxy mergers). In the second scenario, the ring is analogous to the "shells" found around some elliptical galaxies, and would result from a minor merger that took place several Gyr ago. The two interpretations differ in several ways. Tidal arcs are expected to span a limited longitude range; may carry a substantial fraction of the original mass of the satellite; should exhibit a significant velocity gradient with Galactic longitude; and are in all likelihood asymmetric above and below the plane of the disk. Shells, on the other hand, may comprise at most a small fraction of the original mass of the satellite and, due to their more relaxed state, ought to be symmetric above and below the plane, with no discernible velocity gradients across the structure. If confirmed as a tidal feature, the ring discovered by SDSS in the outer Galactic disk would strengthen the view-supported by numerical simulations-that minor mergers have played a significant role building up not only the stellar halo, but also the disk components of the Galaxy.

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A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES: Where Are the Very Metal-poor Stars?

by Amina Helmi and Bruno Letarte

The Astrophysical Journal, 2006

As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have mea... more As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph): Sculptor, Sextans, Fornax and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early Universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] ∼ −3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

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