arXiv subjects

Amina Helmi

Publications and source records attributed to Amina Helmi.

At least 55 records · Page 3Linked to original sources

arXiv2020

Time evolution of gaps in stellar streams in axisymmetric Stäckel potentials

When a subhalo interacts with a cold stellar stream it perturbs its otherwise nearly smooth distribution of stars, and this leads to the creation of a gap. The properties of such gaps depend on the parameters of the interaction. Their characterisation could thus lead to the determination of the mass spectrum of the perturbers and possibly reveal the existence of dark subhalos orbiting the Milky Way. Our goal is to construct a fully analytical model of the formation and evolution of gaps embedded in streams orbiting in a realistic Milky Way potential. To this end, we extend the model of Helmi & Koppelman (2016) for spherical potentials, and predict the properties of gaps in streams evolving in axisymmetric Stäckel potentials. We make use of action-angles and their simple behaviour to calculate the divergence of initially nearby orbits slightly perturbed by the interaction with a subhalo. Our model, corroborated by N-body experiments, predicts that the size of a gap grows linearly with time. We obtain analytical expressions for the dependencies of the growth rate on the orbit of the stream, the properties of the subhalo (mass, scale-radius), and the geometry of the encounter (relative velocity, impact parameter). We find that the density at the centre of the gap decreases with time as a power-law in the same way as the density of a stream. This results in the density-contrast between a pristine and a perturbed stream on the same orbit asymptotically reaching a constant value dependent only on the encounter parameters. We find that at a fixed age, smallish gaps are sensitive mostly to the mass of the subhalo, while gaps formed by subhalo flybys with a low relative velocity, or when the stream and subhalo move parallel, are degenerate to the encounter parameters.

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arXiv2020

Frequencies, chaos and resonances: a study of orbital parameters of nearby thick disc and halo stars

We study the distribution of nearby thick disc and halo stars in subspaces defined by their characteristic orbital parameters. Our aim is to establish the origin of the structure reported in particular, in the $R_{\rm max}-z_{\rm max}$ space. To this end, we compute the orbital parameters and frequencies of stars for a generic and for a Stäckel Milky Way potential. We find that for both the thick disc and halo populations very similar prominent substructures are apparent for the generic Galactic potential, while no substructure is seen for the Stäckel model. This indicates that the origin of these features is not merger-related, but due to non-integrability of the generic potential. This conclusion is strengthened by our frequency analysis of the orbits of stars, which reveals the presence of prominent resonances, with $\sim 30\%$ of the halo stars associated to resonance families. In fact, the stars in resonances define the substructures seen in the spaces of characteristic orbital parameters. Furthermore, we find that some stars in our sample and in debris streams are on the same resonance as the Sagittarius dwarf, suggesting this system has also influenced the distribution of stars in the Galactic thick disc and halo components. Our study constitutes a step towards disentangling the imprint of merger debris from substructures driven by internal dynamics. Given their prominence, these resonant-driven overdensities could potentially be useful to constrain the exact form of the Galactic potential.

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arXiv2020

Streams, substructures and the early history of the Milky Way

The advent of Gaia's 2nd data release in combination with large spectroscopic surveys are revolutionizing our understanding of the Galaxy. Thanks to these and the knowledge accumulated thus far, a more mature picture of the evolution of the early Milky Way is emerging: * Two of the traditional Galactic components, i.e. the stellar halo and the thick disk, appear to be intimately linked: stars with halo-like kinematics originate in similar proportions, from a "heated" (thick) disk and from debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and probably completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ~ 1.8. * The Helmi streams, Sequoia, and Thamnos are amongst the newly uncovered or better characterized merger events. Knowledge of their progenitor's properties, star formation and chemical histories is still incomplete. * Debris' from different objects often overlap in phase-space. A task for the next years will be to use spectroscopic surveys for chemical labelling and to disentangle events from one another using dimensions other than only phase-space, metallicity or [alpha/Fe]. * These surveys will also provide line-of-sight velocities missing for faint stars and more accurate distance determinations for distant objects. The resulting samples of stars will cover a much wider volume of the Galaxy allowing, for example, linking kinematic substructures in the inner halo to spatial overdensities in the outer halo. * All the results obtained so far are in-line with expectations of current cosmological models. Yet, tailored hydrodynamical simulations as well as "constrained" cosmological simulations are needed to push our knowledge of the assembly of the Milky Way back to the earliest times. [abridged]

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arXiv2020

The heavy-elements heritage of the falling sky

Recent dynamical analysis based on Gaia data have revealed major accretion events in Milky Way's history. Nevertheless, our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondently oldest accreted stars remains challenging. Contrary to alpha-elements, neutron-capture elements present unexplained large abundance spreads for low metallicity stars, that could result from a mixture of formation sites. We have analysed the abundances of yttrium, europium, magnesium and iron in Milky Way satellite galaxies, field halo stars and globular clusters. The chemical information has been complemented with orbital parameters based on Gaia data. In particular, the orbit's average inclination has been considered. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe] turnovers for satellite galaxies of different masses reveals that higher luminosity systems, for which the [Mg/Fe] abundance declines at higher metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H] regime between -2.25 and -1.25 dex. In addition, the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y/Eu] abundance spread. [Y/Eu] under-abundances typical of protracted chemical evolutions, are preferentially observed in polar-like orbits, pointing to a possible anisotropy in the accretion processes. Our results strongly suggest that the observed [Y/Eu] abundance spread in the Milky Way halo could result from a mixture of systems with different masses. They also highlight that both nature and nurture are relevant to the Milky Way formation, since its primordial epochs, opening new pathways for chemical diagnostics of our Galaxy building up.

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arXiv2020

A massive mess: When a large dwarf and a Milky Way-like galaxy merge

Circa 10 billion years ago the Milky Way merged with a massive satellite, Gaia-Enceladus. To gain insight into the properties of its debris we analyse in detail the suite of simulations from Villalobos & Helmi (2008), which includes an experiment that produces a good match to the kinematics of nearby halo stars inferred from Gaia data. We compare the kinematic distributions of stellar particles in the simulations and study the distribution of debris in orbital angular momentum, eccentricity and energy, and its relation to the mass-loss history of the simulated satellite. We confirm that Gaia-Enceladus probably fell in on a retrograde, 30$^\circ$ inclination orbit. We find that while 75% of the debris in our preferred simulation has large eccentricity ($> 0.8$), roughly 9% has eccentricity smaller than 0.6. Star particles lost early have large retrograde motions, and a subset of these have low eccentricity. Such stars would be expected to have lower metallicities as they stem from the outskirts of the satellite, and hence naively they could be confused with debris associated with a separate system. These considerations seem to apply to some of the stars from the postulated Sequoia galaxy. When a massive discy galaxy merges, it leaves behind debris with a complex phase-space structure, a large range of orbital properties, and a range of chemical abundances. Observationally, this results in substructures with very different properties, which can be misinterpreted as implying independent progeny. Detailed chemical abundances of large samples of stars and tailored hydrodynamical simulations are critical to resolving such conundrums.

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arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances

We present part 2 of the 6th and final Data Release (DR6 or FDR) of the Radial Velocity Experiment (RAVE), a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A) and span the complete time frame from the start of RAVE observations on 12 April 2003 to their completion on 4 April 2013. In the second of two publications, we present the data products derived from 518387 observations of 451783 unique stars using a suite of advanced reduction pipelines focussing on stellar atmospheric parameters, in particular purely spectroscopically derived stellar atmospheric parameters (Teff, log(g), and the overall metallicity), enhanced stellar atmospheric parameters inferred via a Bayesian pipeline using Gaia DR2 astrometric priors, and asteroseismically calibrated stellar atmospheric parameters for giant stars based on asteroseismic observations for 699 K2 stars. In addition, we provide abundances of the elements Fe, Al, and Ni, as well as an overall [alpha/Fe] ratio obtained using a new pipeline based on the GAUGUIN optimization method that is able to deal with variable signal-to-noise ratios. The RAVE DR6 catalogs are cross matched with relevant astrometric and photometric catalogs, and are complemented by orbital parameters and effective temperatures based on the infrared flux method. The data can be accessed via the RAVE Web site (http://rave-survey.org) or the Vizier database.

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arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- I: Survey Description, Spectra and Radial Velocities

The Radial Velocity Experiment (RAVE) is a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A). The 6th and final data release (DR6 or FDR) is based on 518387 observations of 451783 unique stars. RAVE observations were taken between 12 April 2003 and 4 April 2013. Here we present the genesis, setup and data reduction of RAVE as well as wavelength-calibrated and flux-normalized spectra and error spectra for all observations in RAVE DR6. Furthermore, we present derived spectral classification and radial velocities for the RAVE targets, complemented by cross matches with Gaia DR2 and other relevant catalogs. A comparison between internal error estimates, variances derived from stars with more than one observing epoch and a comparison with radial velocities of Gaia DR2 reveals consistently that 68% of the objects have a velocity accuracy better than 1.4 km/s, while 95% of the objects have radial velocities better than 4.0 km/s. Stellar atmospheric parameters, abundances and distances are presented in subsequent publication. The data can be accessed via the RAVE Web (http://rave-survey.org) or the Vizier database.

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arXiv2020

Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way

Recent studies using Gaia DR2 have identified a massive merger in the history of the Milky Way (MW) whose debris is markedly radial and counterrotating. This event, known as the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo. We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within $Λ$CDM. From $\sim$150 MW analogs, $\sim 80 \%$ have experienced at least one merger of similar mass and infall time as GE/GS. Within this sample, 37 have debris as radial as that of the GE/GS, which we dub the Ancient Radial Mergers (ARMs). Counterrotation is not rare among ARMs, with $43 \%$ having $>40 \%$ of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris, given its large $β$ and its substantial contribution to the stellar halo, is more difficult to reproduce. The median radius of ARM debris is r$_{*,deb}\simeq 45$kpc, while GE/GS is thought to be mostly contained within $r\sim 30$ kpc. For most MW analogs, a few mergers are required to build the inner stellar halo, and ARM debris only accounts for $\sim 12 \%$ of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analog. Interestingly, this merger deposits a significant number of stars (M$_*\simeq1.5 \times 10^9 M_\odot$) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.

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arXiv2019

Stellar 3-D kinematics in the Draco dwarf spheroidal galaxy

Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy. Methods. By combining first-epoch $Hubble$ $Space$ $Telescope$ observations and second-epoch $Gaia$ Data Release 2 positions, we measured the proper motions of $149$ sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of $81$ red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of $45$ Draco members with high-precision and accurate 3D motions, which we present as a table in this paper. Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of $\sim120$ pc from the centre of Draco to be $σ_{R} =11.0^{+2.1}_{-1.5}$ km/s, $σ_{T}=9.9^{+2.3}_{-3.1}$ km/s and $σ_{LOS}=9.0^{+1.1}_{-1.1}$ km/s in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy $β=0.25^{+0.47}_{-1.38}$ at $r \gtrsim120$ pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than $\approx 1/3$ of the escape velocity of the system. In this case, we constrain the maximum circular velocity $V_{max}$ of Draco to be in the range of $10.2-17.0$ km/s. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only. Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming $Gaia$ releases.

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arXiv2019

Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history

Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with [Fe/H] $<-1$ have more retrograde motions than those with [Fe/H] $>-1$. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick-disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high-energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot be fully settled yet. In the low-energy part of the halo we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low inclination, mildly eccentric retrograde orbits, moving at $v_ϕ\approx-150$ km/s, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data it remains challenging to piece together all the fragments found in the retrograde halo. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.

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arXiv2019

Origin of the system of globular clusters in the Milky Way

$Context$. The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided by the $Gaia$ mission. It is likely that the globular cluster system of our Galaxy has followed a similarly intricate formation path. $Aims$. To constrain this formation path, we explore the link between the globular clusters and the known merging events that the Milky Way has experienced. $Methods$. To this end, we combined the kinematic information provided by $Gaia$ for almost all Galactic clusters, with the largest sample of cluster ages available after carefully correcting for systematic errors. To identify clusters with a common origin we analysed their dynamical properties, particularly in the space of integrals of motion. $Results$. We find that about 40% of the clusters likely formed in situ. A similarly large fraction, 35%, appear to be possibly associated to known merger events, in particular to $Gaia$-Enceladus (19%), the Sagittarius dwarf galaxy (5%), the progenitor of the Helmi streams (6%), and to the Sequoia galaxy (5%), although some uncertainty remains due to the degree of overlap in their dynamical characteristics. Of the remaining clusters, 16% are tentatively associated to a group with high binding energy, while the rest are all on loosely bound orbits and likely have a more heterogeneous origin. The resulting age-metallicity relations are remarkably tight and differ in their detailed properties depending on the progenitor, providing further confidence on the associations made. $Conclusions$. We provide a table listing the likely associations. Improved kinematic data by future Gaia data releases and especially a larger, systematic error-free sample of cluster ages would help to further solidify our conclusions.

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arXiv2019

A Gaia-Enceladus Analog in the EAGLE Simulation: Insights into the Early Evolution of the Milky Way

We identify a simulated Milky Way analog in the EAGLE suite of cosmological hydrodynamical simulations. This galaxy not only shares similar global properties as the Milky Way, but was specifically selected because its merger history resembles that currently known for the Milky Way. In particular we find that this Milky Way analog has experienced its last significant merger (with a stellar mass ratio $\sim 0.2$) at $z\sim 1.2$. We show that this merger affected both the dynamical properties of the stars present at the time, contributing to the formation of a thick disk, and also leading to a significant increase in the star formation rate of the host. This object is thus particularly suitable for understanding the early evolutionary history of the Milky Way. It is also an ideal candidate for re-simulation with much higher resolution as this would allow addressing a plethora of interesting questions such as, for example, the specific distribution of dark matter near the Sun.

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arXiv2019

The tilt of the velocity ellipsoid in the Milky Way with Gaia DR2

The velocity distribution of stars is a sensitive probe of the gravitational potential of the Galaxy, and hence of its dark matter distribution. In particular, the shape of the dark halo (e.g. spherical, oblate, or prolate) determines velocity correlations, and different halo geometries are expected to result in measurable differences. Here we explore and interpret the correlations in the $(v_R, v_z)$-velocity distribution as a function of position in the Milky Way. We selected a high-quality sample of stars from the Gaia DR2 catalogue and characterised the orientation of the velocity distribution or tilt angle over a radial distance range of $[4-13]~$kpc and up to $3.5~$kpc away from the Galactic plane while taking into account the effects of the measurement errors. We find that the tilt angles change from spherical alignment in the inner Galaxy ($R\sim4~$kpc) towards more cylindrical alignments in the outer Galaxy ($R\sim11~$kpc) when using distances that take a global zero-point offset in the parallax of $-29~μ$as. However, if the amplitude of this offset is underestimated, then the inferred tilt angles in the outer Galaxy only appear shallower and are intrinsically more consistent with spherical alignment for an offset as large as $-54~μ$as. We further find that the tilt angles do not seem to strongly vary with Galactic azimuth and that different stellar populations depict similar tilt angles. Therefore we introduce a simple analytic function that describes the trends found over the full radial range. Since the systematic parallax errors in Gaia DR2 depend on celestial position, magnitude, and colour in complex ways, it is not possible to fully correct for them. Therefore it will be particularly important for dynamical modelling of the Milky Way to thoroughly characterise the systematics in astrometry in future Gaia data releases.

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arXiv2019

Axisymmetric Schwarzschild models of an isothermal axisymmetric mock dwarf spheroidal galaxy

We test the ability of Schwarzschild's orbit superposition method in measuring the mass content, scale radius and shape of a flattened dwarf spheroidal galaxy. Until now, most dynamical model efforts have assumed that dwarf spheroidal galaxies and their host halos are spherical. We use an Evans model (1993) to construct an isothermal mock galaxy whose properties somewhat resemble those of the Sculptor dwarf spheroidal galaxy. This mock galaxy contains flattened luminous and dark matter components, resulting in a logarithmic profile for the global potential. We have tested how well our Schwarzschild method could constrain the characteristic parameters of the system for different sample sizes, and also if the functional form of the potential was unknown. When assuming the true functional form of the potential, the Schwarzschild modelling technique is able to provide an accurate and precise measurement of the characteristic mass parameter of the system and reproduces well the light distribution and the stellar kinematics of our mock galaxy. When assuming a different functional form for the potential, such as a flattened NFW profile, we also constrain the mass and scale radius to their expected values. However in both cases, we find that the flattening parameter remains largely unconstrained. This is likely because the information content of the velocity dispersion on the geometric shape of the potential is too small, since $σ$ is constant across our mock dSph. Our results using Schwarzschild's method indicate that the mass enclosed can be derived reliably, even if the flattening parameter is unknown, and already for samples containing 2000 line-of-sight radial velocities, such as those currently available. Further applications of the method to more general distribution functions of flattened systems are needed to establish how well the flattening of dSph dark halos can be determined.

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arXiv2019

VLT/FLAMES high-resolution chemical abundances in Sculptor: a textbook dwarf spheroidal galaxy

We present detailed chemical abundances for 99 red-giant branch stars in the centre of the Sculptor dwarf spheroidal galaxy, obtained from high-resolution VLT/FLAMES spectroscopy. This includes abundances of Li (detected in 1 out of 99 stars), Na, $α$-elements (O, Mg, Si, Ca Ti), iron-peak elements (Sc, Cr, Fe, Co, Ni, Zn), r- and s-process elements (Ba, La, Nd, Eu). The sample, covering -2.3< [Fe/H]<-0.9, populates the whole metallicity distribution of the galaxy with the exception of the very low metallicity tail. There is a marked decrease in [$α$/Fe] over our sample, from the Galactic halo plateau value at low [Fe/H] and then, after a `knee', a decrease to sub-solar [$α$/Fe] at high [Fe/H]. This is consistent with products of core-collapse supernovae dominating at early times, followed by the onset of supernova type Ia as early as 12 Gyrs ago. The products from low-mass asymptotic giant branch stars, as traced by the s-process, also participate in the chemical evolution of Sculptor with a timescale comparable to that of supernovae type Ia. The r-process is consistent with having no time delay relative to core-collapse SN at the later stages of the chemical evolution in Sculptor. We derive empirical constraints on the relative importance of supernovae type II and Ia to the nucleosynthesis of individual iron-peak and $α$-elements. The most important contribution of supernovae type Ia is to the iron-peak elements: Fe, Cr and Mn; but there is also a modest but non-negligible contribution to both the heavier $α$-elements: S, Ca and Ti, and some of the iron-peak elements: Sc and Co. We see only very small or no contribution to O, Mg, Ni and Zn from supernovae type Ia in Sculptor. The observed chemical abundances in Sculptor show no evidence of a significantly different initial mass function, compared to that of the Milky Way.

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arXiv2019

The R-Process Alliance: Discovery of a low-alpha, r-Process-Enhanced Metal-Poor Star in the Galactic Halo

A new moderately r-process-enhanced metal-poor star, RAVE J093730.5-062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937-0626 is likely a horizontal branch star. At [Fe/H] = -1.86, J0937-0626 is found to have subsolar [X/Fe] ratios for nearly every light, alpha, and Fe-peak element. The low [alpha/Fe] ratios can be explained by an ~0.6 dex excess of Fe; J0937-0626 is therefore similar to the subclass of "iron-enhanced" metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = -2.5 suggests that J0937-0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937-0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937-0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937-0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.

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arXiv2018

Mass and shape of the Milky Way's dark matter halo with globular clusters from Gaia and Hubble

We estimate the mass of the inner ($<20$ kpc) Milky Way and the axis ratio of its dark matter halo using globular clusters as tracers. At the same time, we constrain the phase-space distribution of the globular cluster system. We use the Gaia DR2 catalogue of 75 globular clusters' proper motions and recent measurements of the proper motions of another 20 distant clusters obtained with the Hubble Space Telescope. We describe the globular cluster system with a 2-component distribution function (DF), with a flat, rotating disc and a rounder, more extended halo. While fixing the Milky Way's disc and bulge, we let the mass and shape of the dark matter halo and we fit these two parameters, together with other six describing the DF, with a Bayesian method. We find the mass of the Galaxy within 20 kpc to be $M(<20{\,\rm kpc})=1.91^{+0.18}_{-0.17} \times 10^{11} M_\odot$, of which $M_{\rm DM}(<20{\,\rm kpc})=1.37^{+0.18}_{-0.17}\times 10^{11}M_\odot$ is in dark matter, and the density axis ratio of the dark matter halo to be $q=1.30 \pm 0.25$. This implies a virial mass $M_{\rm via} = 1.3 \pm 0.3 \times 10^{12} M_\odot$. Our analysis rules out oblate ($q<0.8$) and strongly prolate halos ($q>1.9$) with 99\% probability. Our preferred model reproduces well the observed phase-space distribution of globular clusters and has a disc component that closely resembles that of the Galactic thick disc. The halo component follows a power-law density profile $ρ\propto r^{-3.3}$, has a mean rotational velocity of $V_{\rm rot}\simeq -14\,\rm km\,s^{-1}$ at 20 kpc, and has a mildly radially biased velocity distribution ($β\simeq 0.2 \pm 0.07$, fairly constant outside 15 kpc). We also find that our distinction between disc and halo clusters resembles, although not fully, the observed distinction in metal-rich ([Fe/H]$>-0.8$) and metal-poor ([Fe/H]$\leq-0.8$) cluster populations.

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arXiv2018

White dwarfs in the building blocks of the Galactic spheroid

The galactic halo likely grew over time in part by assembling smaller galaxies, the so-called building blocks. We investigate if the properties of these building blocks are reflected in the halo white dwarf (WD) population in the Solar neighborhood. Furthermore, we compute the halo WD luminosity functions (WDLFs) for four major building blocks of five cosmologically motivated stellar haloes. We couple the SeBa binary population synthesis model to the Munich-Groningen semi-analytic galaxy formation model, applied to the high-resolution Aquarius dark matter simulations. Although the semi-analytic model assumes an instantaneous recycling approximation, we model the evolution of zero-age main sequence stars to WDs, taking age and metallicity variations of the population into account. Although the majority of halo stars is old and metal-poor and therefore the WDs in the different building blocks have similar properties (including present-day luminosity), we find in our models that the WDs originating from building blocks that have young and/or metal-rich stars can be distinguished from WDs that were born in other building blocks. In practice however, it will be hard to prove that these WDs really originate from different building blocks, as the variations in the halo WD population due to binary WD mergers result in similar effects. The five joined stellar halo WD populations that we modelled result in WDLFs that are very similar to each other. We find that simple models with a Kroupa or Salpeter initial mass function (IMF) fit the observed luminosity function slightly better, since the Chabrier IMF is more top-heavy, although this result is dependent on our choice of the stellar halo mass density in the Solar neighborhood.

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