ABSTRACT
We characterize the Gaia–Enceladus–Sausage kinematic structure recently discovered in the Galactic halo using photometric metallicities from the SkyMapper survey, and kinematics from Gaia ...radial velocities measurements. By examining the metallicity distribution functions (MDFs) of stars binned in kinematic/action spaces, we find that the $\sqrt{J_R}$ versus Lz space allows for the cleanest selection of Gaia–Enceladus–Sausage stars with minimal contamination from disc or halo stars formed in situ or in other past mergers. Stars with $30 \le \sqrt{J_R} \le 50$ (kpc km s−1)1/2 and −500 ≤ Lz ≤ 500 kpc km s−1 have a narrow MDF centred at Fe/H = −1.17 dex with a dispersion of 0.34 dex. This Fe/H estimate is more metal-rich than literature estimates by 0.1−0.3 dex. Based on the MDFs, we find that selection of Gaia–Enceladus–Sausage stars in other kinematic/action spaces without additional population information leads to contaminated samples. The clean Gaia–Enceladus–Sausage sample selected according to our criteria is slightly retrograde and lies along the blue sequence of the high VT halo colour magnitude diagram dual sequence. Using a galaxy mass–metallicity relation derived from cosmological simulations and assuming a mean stellar age of 10 Gyr, we estimate the mass of the Gaia–Enceladus–Sausage progenitor satellite to be 108.85–9.85 M⊙, which is consistent with literature estimates based on disc dynamic and simulations. Additional information on detailed abundances and ages would be needed for a more sophisticated selection of purely Gaia–Enceladus–Sausage stars.
ABSTRACT
Identifying stars found in the Milky Way as having formed in situ or accreted can be a complex and uncertain undertaking. We use Gaia kinematics and APOGEE elemental abundances to select ...stars belonging to the Gaia-Sausage-Enceladus (GSE) and Sequoia accretion events. These samples are used to characterize the GSE and Sequoia population metallicity distribution functions, elemental abundance patterns, age distributions, and progenitor masses. We find that the GSE population has a mean Fe/H ∼ −1.15 and a mean age of 10–12 Gyr. GSE has a single sequence in Mg/Fe versus Fe/H consistent with the onset of SN Ia Fe contributions and uniformly low Al/Fe of ∼−0.25 dex. The derived properties of the Sequoia population are strongly dependent on the kinematic selection. We argue the selection with the least contamination is Jϕ/Jtot < −0.6 and (Jz − JR)/Jtot < 0.1. This results in a mean Fe/H ∼ −1.3 and a mean age of 12–14 Gyr. The Sequoia population has a complex elemental abundance distribution with mainly high-Mg/Fe stars. We use the GSE Al/Fe versus Mg/H abundance distribution to inform a chemically based selection of accreted stars, which is used to remove possible contaminant stars from the GSE and Sequoia samples.
ABSTRACT
Using the VINTERGATAN cosmological zoom simulation, we explore the contributions of the in situ and accreted material, and the effect of galaxy interactions and mergers in the assembly of a ...Milky Way-like galaxy. We find that the initial growth phase of galaxy evolution, dominated by repeated major mergers, provides the necessary physical conditions for the assembly of a thick, kinematically hot disc populated by high-α/Fe stars, formed both in situ and in accreted satellite galaxies. We find that the diversity of evolutionary tracks followed by the simulated galaxy and its progenitors leads to very little overlap of the in situ and accreted populations for any given chemical composition. At a given age, the spread in α/Fe abundance ratio results from the diversity of physical conditions in VINTERGATAN and its satellites, with an enhancement in α/Fe found in stars formed during starburst episodes. Later, the cessation of the merger activity promotes the in situ formation of stars in the low-α/Fe regime, in a radially extended, thin and overall kinematically colder disc, thus establishing chemically bimodal thin and thick discs, in line with observations. We draw links between notable features in the Fe/H-α/Fe plane with their physical causes, and propose a comprehensive formation scenario explaining self-consistently, in the cosmological context, the main observed properties of the Milky Way.
ABSTRACT
Using the cosmological zoom simulation VINTERGATAN, we present a new scenario for the onset of star formation at the metal-poor end of the low-α/Fe sequence in a Milky Way-like galaxy. In ...this scenario, the galaxy is fuelled by two distinct gas flows. One is enriched by outflows from massive galaxies, but not the other. While the former feeds the inner galactic region, the latter fuels an outer gas disc, inclined with respect to the main galactic plane, and with a significantly poorer chemical content. The first passage of the last major merger galaxy triggers tidal compression in the outer disc, which increases the gas density and eventually leads to star formation, at a metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of the low-α/Fe sequence. These in situ stars have halo-like kinematics, similar to what is observed in the Milky Way, due to the inclination of the outer disc that eventually aligns with the inner one via gravitational torques. We show that this tilting disc scenario is likely to be common in Milky Way-like galaxies. This process implies that the low-α/Fe sequence is populated in situ, simultaneously from two formation channels, in the inner and the outer galaxy, with distinct metallicities. This contrasts with purely sequential scenarios for the assembly of the Milky Way disc and could be tested observationally.
ABSTRACT
We provide a detailed map of the ages and metallicities of turn-off stars in the Milky Way disc based on data from GALAH DR3 and Gaia EDR3. From this map, we identify previously undetected ...features in the age–metallicity distribution of disc stars and interpret these results as indicating a three-phase formation history of the Milky Way. In the first phase, inner disc stars form along a single age–metallicity sequence and are today kinematically hot. The end of this phase is marked by a local minimum in the inner disc age distribution 10 Gyr ago. At this time, we find the stellar populations to transition from high to low alpha-element abundances and from high to low vertical velocity dispersion. In the second phase, stars form across the disc with outwardly decreasing metallicity. In this phase, inner disc stars form at supersolar metallicities in a continuation of the early age–metallicity relation, while outer disc stars begin forming at metallicities at least 0.5 dex lower. Finally, the third phase is associated with a recent burst of star formation across the local disc marked by a local minimum in the age–metallicity distribution 4–6 Gyr ago. Future quantitative comparisons between the observed age–metallicity distribution and those of simulated galaxies could help constrain the processes driving each of the star formation phases.
Abstract
We study the possibility that the Milky Ways’ cool stellar disk includes mergers with ancient stars. Galaxies are understood to form in a hierarchical manner, where smaller (proto-)galaxies ...merge into larger ones. Stars in galaxies, like the Milky Way, contain in their motions and elemental abundance tracers of past events and can be used to disentangle merger remnants from stars that formed in the main galaxy. The merger history of the Milky Way is generally understood to be particularly easy to study in the stellar halo. The advent of the ESA astrometric satellite Gaia has enabled the detection of completely new structures in the halo such as the Gaia-Enceladus-Sausage. However, simulations also show that mergers may be important for the build-up of the cool stellar disks. Combining elemental abundances for ∼100 giant branch stars from APOGEE DR17 and astrometric data from Gaia we use elemental abundance ratios to find a hitherto unknown, old stellar component in the cool stellar disk in the Milky Way. We further identify a small sample of RR Lyrae variables with disk kinematics that also show the same chemical signature as the accreted red giant stars in the disk. These stars allow us to date the stars in the accreted component. We find that they are exclusively old.
ABSTRACT
Spectroscopic surveys of the Milky Way’s stars have revealed spatial, chemical, and kinematical structures that encode its history. In this work, we study their origins using a cosmological ...zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at z ∼ 1.5, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-α/Fe gas disc around the inner, metal-rich galaxy containing the old high-α/Fe stars. This event leads to a bimodality in α/Fe over a range of Fe/H. A detailed analysis of how the galaxy evolves since z ∼ 1 is presented. We demonstrate the way in which inside-out growth shapes the radial surface density and metallicity profile and how radial migration preferentially relocates stars from the inner disc to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scale heights with stellar age, which together with disc flaring explains several trends observed in the Milky Way, including shallower radial Fe/H profiles above the mid-plane. We show how the galaxy formation scenario imprints non-trivial mappings between structural associations (i.e. thick and thin discs), velocity dispersions, α-enhancements, and ages of stars; e.g. the most metal-poor stars in the low-α/Fe sequence are found to have a scale height comparable to old high-α/Fe stars. Finally, we illustrate how at low spatial resolution, comparable to the thickness of the galaxy, the proposed pathway to distinct sequences in α/Fe–Fe/H cannot be captured.
ABSTRACT
Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus ...(GSE), appears to be an early ‘building block’ given its virial mass $\gt 10^{10}\, \mathrm{M_\odot }$ at infall (z ∼ 1−3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-α abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via Na/Fe versus Mg/Mn in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 \lt \sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} \lt 55$, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for example mean $\sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} =$$26_{-14}^{+9}$. We find only $(29\pm 1){{\ \rm per\ cent}}$ of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
ABSTRACT
Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved ...stars through isochrone matching. This work presents M/H–age and α/M–age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the M/H–age relation as a function of both Galactocentric radius and distance from the disc mid-plane. The M/H–age relations support claims that radial migration has a significant effect in the plane of the disc. Using the M/H bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be −0.059 ± 0.010 dex kpc−1, in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the M/H–α/M distribution of the solar neighbourhood suggests that the high-M/H stars are not an evolutionary extension of the low-α sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.
Data from the SDSS-IV/Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have been released as part of SDSS Data Releases 13 (DR13) and 14 (DR14). These include high-resolution H-band ...spectra, radial velocities, and derived stellar parameters and abundances. DR13, released in 2016 August, contained APOGEE data for roughly 150,000 stars, and DR14, released in 2017 August, added about 110,000 more. Stellar parameters and abundances have been derived with an automated pipeline, the APOGEE Stellar Parameter and Chemical Abundance Pipeline (ASPCAP). We evaluate the performance of this pipeline by comparing the derived stellar parameters and abundances to those inferred from optical spectra and analysis for several hundred stars. For most elements-C, Na, Mg, Al, Si, S, Ca, Cr, Mn, Ni-the DR14 ASPCAP analyses have systematic differences with the comparisons samples of less than 0.05 dex (median), and random differences of less than 0.15 dex (standard deviation). These differences are a combination of the uncertainties in both the comparison samples as well as the ASPCAP analysis. Compared to the references, magnesium is the most accurate alpha-element derived by ASPCAP, and shows a very clear thin/thick disk separation, while nickel is the most accurate iron-peak element (besides iron itself).
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