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Register a new userSelecting accreted populations: metallicity, elemental abundances, and ages of the Gaia-Sausage-Enceladus and Sequoia populations
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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] $sim -1.15$ and a mean age of $10-12$ Gyr. GSE has a single sequence in [Mg/Fe] vs [Fe/H] consistent with the onset of SN Ia Fe contributions and uniformly low [Al/Fe] of $sim -0.25$ dex. The derived properties of the Sequoia population are strongly dependent on the kinematic selection. We argue the selection with the least contamination is $J_{phi}/J_{mbox{tot}} < -0.6$ and $(J_z - J_R)/J_{mbox{tot}} < 0.1$. This results in a mean [Fe/H] $sim -1.3$ 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] vs [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.
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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 $sqrt{J_R}$ vs $L_z$ 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 $30 leq sqrt{J_R} leq 50$ (kpc km s$^{-1})^{1/2}$ and $-500 leq L_z leq 500$ kpc km s$^{-1}$ have a narrow MDF centered 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 $V_T$ halo CMD 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 $10^{8.85-9.85}$ M$_{odot}$, 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.
The low-metallicity, kinematically interesting dwarf stars studied by Stephens & Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry, and updated model atmospheres and atomic line data. New stellar parameters are determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar Evolution Database isochrones. These are in excellent agreement with spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$; however, large disagreements are found for stars with [Fe/H]$le-2$, with offsets as large as $Delta$T$_{rm eff}sim+500$ K and $Delta$log,$gsim+1.0$. A subset of six stars (test cases) are analysed ab initio using high resolution spectra with Keck HIRES and Gemini GRACES. This sub-sample is found to include two $alpha$-challenged dwarf stars, suggestive of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the entire SB02 sample are re-determined using textit{Gaia} DR2 data. We find 11 stars that are dynamically coincident with the textit{Gaia}-Sausage accretion event and another 17 with the textit{Gaia}-Sequoia event in action space. Both associations include low-mass, metal-poor stars with isochrone ages older than 10 Gyr. Two dynamical subsets are identified within textit{Gaia}-Sequoia. When these subsets are examined separately, a common knee in [$alpha$/Fe] is found for the textit{Gaia}-Sausage and low orbital energy textit{Gaia}-Sequoia stars. A lower metallicity knee is tentatively identified in the textit{Gaia}-Sequoia high orbital energy stars. If the metal-poor dwarf stars in these samples are true members of the textit{Gaia}-Sausage and textit{Gaia}-Sequoia events, then they present a unique opportunity to probe the earlier, more pristine, star formation histories of these systems.
Context. The TOPoS project has the goal to find and analyse Turn-Off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we have relied on low-resolution spectra from the Sloan Digital Sky Survey. Aims. In this paper we use the metallicity estimates we have obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We make use of SDSS photometry, Gaia photometry and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample is used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows to correct for the selection bias. We select a sub-sample of stars with reliable parallaxes for which we combine the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allows us to characterize the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus able to provide also the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low mass galaxies rather than massive galaxies, such as the GSE progenitor.
We analyse a set of cosmological magneto-hydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called Gaia Sausage found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50$%$ of gas to a merger-induced centrally concentrated starburst that results in the rapid formation of a compact, rotationally supported thick disc that occupies the typical chemical thick disc region of chemical abundance space. We find evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering stars onto less-circular orbits such that their rotation velocity and metallicity positively correlate, thus contributing an additional component that connects the Galactic thick disc to the inner stellar halo. We demonstrate that the level of kinematic heating of the proto-galaxy correlates with the kinematic state of the population before the merger, the progenitor mass and orbital eccentricity of the merger. Furthermore, we show that the mass and time of the merger can be accurately inferred from local stars on counter-rotating orbits.
We present evidence that multiple accretion events are required to explain the origin of the $Gaia$-Sausage and Enceladus (GSE) structures, based on an analysis of dynamical properties of main-sequence stars from the Sloan Digital Sky Survey Data Release 12 and $Gaia$ Data Release 2. GSE members are selected to have eccentricity ($e$) $>$ 0.7 and [Fe/H] $<$ -1.0, and separated into low and high orbital-inclination (LOI/HOI) groups. We find that the LOI stars mainly have $e < 0.9$ and are clearly separable into two groups with prograde and retrograde motions. The LOI stars exhibit prograde motions in the inner-halo region and strong retrograde motions in the outer-halo region. We interpret the LOI stars in these regions to be stars accreted from two massive dwarf galaxies with low-inclination prograde and retrograde orbits, affected to different extents by dynamical friction due to their different orbital directions. In contrast, the majority of the HOI stars have $e > 0.9$, and exhibit a globally symmetric distribution of rotational velocities ($V_{rm phi}$) near zero, although there is evidence for a small retrograde motion for these stars ($V_{rm phi}$ $sim$ -15 $rm{km~s^{-1}}$) in the outer-halo region. We consider these stars to be stripped from a massive dwarf galaxy on a high-inclination orbit. We also find that the LOI and HOI stars on highly eccentric and tangential orbits with clear retrograde motions exhibit different metallicity peaks at [Fe/H] = -1.7 and -1.9, respectively, and argue that they are associated with two low-mass dwarf galaxies accreted in the outer-halo region of the Galaxy.
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