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 ∼0.2) at z ∼ 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.
We investigate the chemical and kinematic properties of the diffuse stellar haloes of six simulated Milky-Way-like galaxies from the Aquarius Project. Binding energy criteria are adopted to define ...two dynamically distinct stellar populations: the diffuse inner and outer haloes, which comprise different stellar subpopulations with particular chemical and kinematic characteristics. Our simulated inner- and outer-halo stellar populations have received contributions from debris stars (formed in subgalactic systems while they were outside the virial radius of the main progenitor galaxies) and endo-debris stars (those formed in gas-rich subgalactic systems inside the dark matter haloes of the main progenitor galaxy). The inner haloes possess an additional contribution from disc-heated stars, in the range ∼3-30 per cent, with a mean of ∼20 per cent. Disc-heated stars might exhibit signatures of kinematical support, in particular among the youngest ones. Endo-debris plus disc-heated stars define the so-called in situ stellar populations. In both the inner- and outer-halo stellar populations, we detect contributions from stars with moderate to low α/Fe ratios, mainly associated with the endo-debris or disc-heated subpopulations. The observed abundance gradients in the inner-halo regions are influenced by both the level of chemical enrichment and the relative contributions from each stellar subpopulation. Steeper abundance gradients in the inner-halo regions are related to contributions from the disc-heated and endo-debris stars, which tend to be found at lower binding energies than debris stars. In the case of the outer-halo regions, although Fe/H gradients are relatively mild, the steeper profiles arise primarily due to contributions from stars formed in more massive satellites, which sink farther into the main halo system, and tend to have higher levels of chemical enrichment and lower energies. Our findings support the existence of (at least) two distinct diffuse stellar halo populations, as suggested by a number of recent observations in the Milky Way and M31. Our results also indicate that a comparison of the range of predicted kinematics, abundance gradients and frequency of α/Fe-deficient stars with observations of these quantities in the Milky Way, M31 and other large spirals can both provide clues to improve the modelling of baryonic physics, and reveal detailed information about their likely history of formation and evolution.
We present a comprehensive study of the chemical properties of the stellar haloes of Milky Way mass galaxies, analysing the transition between the inner to the outer haloes. We find the transition ...radius between the relative dominance of the inner-halo and outer-halo stellar populations to be ∼15-20 kpc for most of our haloes, similar to that inferred for the Milky Way from recent observations. While the number density of stars in the simulated inner-halo populations decreases rapidly with distance, the outer-halo populations contribute about 20-40 per cent in the fiducial solar neighbourhood, in particular at the lowest metallicities. We have determined Fe/H profiles for our simulated haloes; they exhibit flat or mild gradients, in the range −0.002, −0.01 dex kpc−1. The metallicity distribution functions exhibit different features, reflecting the different assembly history of the individual stellar haloes. We find that stellar haloes formed with larger contributions from massive subgalactic systems have steeper metallicity gradients. Very metal-poor stars are mainly contributed to the halo systems by lower mass satellites. There is a clear trend among the predicted metallicity distribution functions that a higher fraction of low-metallicity stars are found with increasing radius. These properties are consistent with the range of behaviours observed for stellar haloes of nearby galaxies.
Abstract
We explore the chemodynamical properties of a sample of very metal-poor (VMP) stars selected from the Hamburg/ESO survey, matched with Gaia EDR3, in the phase space identified by the three ...integrals of motion (
L
z
,
E
,
I
3
). Disk and halo orbits are separated by using the criteria defined in Carollo & Chiba. We found 26 stars with Fe/H ≤ −2.5 possessing disk kinematics, of which 13 are extremely metal-poor. At these metallicities, the number of stars with disk kinematics is three times its retrograde counterpart. In the same range of metallicity we also identified 37 halo stars most tightly bound to the gravitational potential of the progenitor halo. The origins of these stars are investigated by comparing the observational results with simulated galaxies from the Aquarius Project and the IllustrisTNG simulations. We found two mechanisms of formation of VMP stars with disk kinematics: accretion from early satellites (which is dominant), and in situ formation. These stars are very old, with ages >12.5 Gyr (
z
> 5), and they are
α
-enriched. Accretion and in situ formation are also found for the retrograde counterparts, with accretion also being the dominant mode. Contributing accreted satellites have stellar masses in the range 10
6
–10
9
M
☉
and are very gas-rich. The most bound halo stars are the oldest detected, with a median age of ∼13.3 Gyr (
z
∼ 11), and are
α
-enriched. Our finding clearly shows that very old, VMP stars store important information on the first stages of assembly of our Galaxy and its halo.
We find two chemically distinct populations separated relatively cleanly in the Fe/H-Mg/Fe plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities ...) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their X/Fe ratios for the -elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low- and high- halo stars found in previous studies, suggesting that these are samples of the same two populations.
Abstract
We examine the stellar haloes of the Auriga simulations, a suite of 30 cosmological magnetohydrodynamical high-resolution simulations of Milky Way-mass galaxies performed with the ...moving-mesh code arepo. We study halo global properties and radial profiles out to ∼150 kpc for each individual galaxy. The Auriga haloes are diverse in their masses and density profiles, mean metallicity and metallicity gradients, ages, and shapes, reflecting the stochasticity inherent in their accretion and merger histories. A comparison with observations of nearby late-type galaxies shows very good agreement between most observed and simulated halo properties. However, Auriga haloes are typically too massive. We find a connection between population gradients and mass assembly history: galaxies with few significant progenitors have more massive haloes, possess large negative halo metallicity gradients, and steeper density profiles. The number of accreted galaxies, either disrupted or under disruption, that contribute 90 per cent of the accreted halo mass ranges from 1 to 14, with a median of 6.5, and their stellar masses span over three orders of magnitude. The observed halo mass–metallicity relation is well reproduced by Auriga and is set by the stellar mass and metallicity of the dominant satellite contributors. This relationship is found not only for the accreted component but also for the total (accreted + in situ) stellar halo. Our results highlight the potential of observable halo properties to infer the assembly history of galaxies.
We study the chemical properties of the stellar populations in eight simulations of the formation of Milky Way mass galaxies in a Λ cold dark matter universe. Our simulations include metal-dependent ...cooling and an explicitly multiphase treatment of the effects on the gas of cooling, enrichment and supernova feedback. We search for correlations between formation history and chemical abundance patterns. Differing contributions to spheroids and discs from in situ star formation and from accreted populations are reflected in differing chemical properties. Discs have younger stellar populations, with most stars forming in situ and with low α-enhancement from gas which never participated in a galactic outflow. Up to 15 per cent of disc stars can come from accreted satellites. These tend to be α-enhanced, older and to have larger velocity dispersions than the in situ population. Inner spheroids have old, metal-rich and α-enhanced stars which formed primarily in situ, more than 40 per cent from material recycled through earlier galactic winds. Few accreted stars are found in the inner spheroid unless a major merger occurred recently. Such stars are older, more metal-poor and more α-enhanced than the in situ population. Stellar haloes tend to have low metallicity and high α-enhancement. The outer haloes are made primarily of accreted stars. Their mean metallicity and α-enhancement reflect the masses of the disrupted satellites where they formed: more massive satellites typically have higher Fe/H and lower α/Fe. Surviving satellites have distinctive chemical patterns which reflect their extended, bursty star formation histories. These produce lower α-enhancement at given metallicity than in the main galaxy, in agreement with observed trends in the Milky Way.
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