The origin of accreted stellar halo populations in the Milky Way using APOGEE, Gaia, and the EAGLE simulations
Liverpool John Moores University1, University of Virginia2, University of Toronto3, Spanish National Research Council4, University of La Laguna5, University of Utah6, New Mexico State University7, Ohio State University8, University of Texas at Austin9, Andrés Bello National University10, University of Atacama11, Centre national de la recherche scientifique12, University of Concepción13
21 Jan 2019-Monthly Notices of the Royal Astronomical Society (Oxford Academic)-Vol. 482, Iss: 3, pp 3426-3442
TL;DR: In this article, the authors show that 2/3 of nearby halo stars have high orbital eccentricities and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterised by relatively low [Fe/H], [Mg/Fe], [Al/Fe, and [Ni/Fe] abundances.
Abstract: Recent work indicates that the nearby Galactic halo is dominated by the debris from a major accretion event. We confirm that result from an analysis of APOGEE-DR14 element abundances and $\textit{Gaia}$-DR2 kinematics of halo stars. We show that $\sim$2/3 of nearby halo stars have high orbital eccentricities ($e \gtrsim 0.8$), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterised by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high $e$ stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H]$\sim-1.3$, which is also typical of stellar populations from relatively massive dwarf galaxies. Low $e$ stars exhibit no such change of slope within the observed [Fe/H] range and show slightly higher abundances of Mg, Al and Ni. Unlike their low $e$ counterparts, high $e$ stars show slightly retrograde motion, make higher vertical excursions and reach larger apocentre radii. By comparing the position in [Mg/Fe]-[Fe/H] space of high $e$ stars with those of accreted galaxies from the EAGLE suite of cosmological simulations we constrain the mass of the accreted satellite to be in the range $10^{8.5}\lesssim M_*\lesssim 10^{9}\mathrm{M_\odot}$. We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at $z\lesssim1.5$. The exact nature of the low $e$ population is unclear, but we hypothesise that it is a combination of $\textit{in situ}$ star formation, high $|z|$ disc stars, lower mass accretion events, and contamination by the low $e$ tail of the high $e$ population. Finally, our results imply that the accretion history of the Milky Way was quite unusual.
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TL;DR: In this paper, the authors provided dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage, which is the major accretion event that built the stellar halo of the Milky Way galaxy.
Abstract: The Gaia Sausage is the major accretion event that built the stellar halo of the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage. The Sequoia Event provided the bulk of the high energy retrograde stars in the stellar halo, as well as the recently discovered globular cluster FSR 1758. There are up to 6 further globular clusters, including $\omega$~Centauri, as well as many of the retrograde substructures in Myeong et al. (2018), associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is $\sim 5 \times 10^{7} M_\odot$, whilst the total mass is $\sim 10^{10} M_\odot$, as judged from abundance matching or from the total sum of the globular cluster mass. Although clearly less massive than the Sausage, the Sequoia has a distinct chemo-dynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of $\sim0.6$, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by $\sim 0.3$ dex and higher abundance ratios as compared to the Sausage. We conjecture that the Sausage and the Sequoia galaxies may have been associated and accreted at a comparable epoch.
344 citations
Cites background or methods or result from "The origin of accreted stellar halo..."
...For the low eccentricity group, Mackereth et al. (2019) suggested they are likely to be a mixture of in situ halo stars and many smaller accreted materials – which includes the Sequoia debris....
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...With APOGEE DR14 (Abolfathi et al. 2018), Mackereth et al. (2019) showed that halo stars with high eccentricity orbits tend to have lower [Mg/Fe] on average compared to the rest of the halo stars....
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...This was derived in Fattahi et al. (2019) using cosmological hydrodynamical simulations (Grand et al. 2017), and is consistent with the estimates from Mackereth et al. (2019) and Vincenzo et al. (2019)....
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...Our hypothesis is more closely related to the work of Mackereth et al. (2019), who divided halo stars according to eccentricity and showed that the low and high eccentricity groups have different abundance ratios and probably different origin....
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...9, as well as in Mackereth et al. (2019) and Matsuno et al. (2019)....
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TL;DR: In this article, the distribution of globular clusters in the 6d phase space, using both position/velocity and action/angle coordinates, was analyzed using Gaia Data Release 2.
Abstract: We use Gaia Data Release 2 to determine the mean proper motions for 150 Milky Way globular clusters (almost the entire known population), with a typical uncertainty of 0.05 mas/yr limited mainly by systematic errors. Combining them with distance and line-of-sight velocity measurements from the literature, we analyze the distribution of globular clusters in the 6d phase space, using both position/velocity and action/angle coordinates. The population of clusters in the central 10 kpc has a mean rotational velocity reaching 50-80 km/s, and a nearly isotropic velocity dispersion 100-120 km/s, while in the outer galaxy the cluster orbits are strongly radially anisotropic. We confirm a concentration of clusters at high radial action in the outer region of the Galaxy. Finally, we explore a range of equilibrium distribution function-based models for the entire globular cluster system, and the information they provide about the potential of the Milky Way. The dynamics of clusters is best described by models with the circular velocity between 10 and 50 kpc staying in the range 210-240 km/s.
254 citations
Max Planck Society1, Australian National University2, University of Sydney3, University of Ljubljana4, Uppsala University5, Stockholm University6, University of New South Wales7, Monash University8, Macquarie University9, Aarhus University10, Leibniz Institute for Astrophysics Potsdam11, University of Southern Queensland12, International Space Science Institute13, Lund University14, University of Western Australia15, Kapteyn Astronomical Institute16, University of Hertfordshire17, Swinburne University of Technology18, Johns Hopkins University19, Columbia University20, York University21
TL;DR: In this paper, the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab1242
Abstract: © 2021 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab1242
235 citations
TL;DR: The most recent data release from the Sloan Digital Sky Surveys (SDSS-IV) is DR17 as discussed by the authors , which contains the complete release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey.
Abstract: This paper documents the seventeenth data release (DR17) from the Sloan Digital Sky Surveys; the fifth and final release from the fourth phase (SDSS-IV). DR17 contains the complete release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, which reached its goal of surveying over 10,000 nearby galaxies. The complete release of the MaNGA Stellar Library (MaStar) accompanies this data, providing observations of almost 30,000 stars through the MaNGA instrument during bright time. DR17 also contains the complete release of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) survey which publicly releases infra-red spectra of over 650,000 stars. The main sample from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), as well as the sub-survey Time Domain Spectroscopic Survey (TDSS) data were fully released in DR16. New single-fiber optical spectroscopy released in DR17 is from the SPectroscipic IDentification of ERosita Survey (SPIDERS) sub-survey and the eBOSS-RM program. Along with the primary data sets, DR17 includes 25 new or updated Value Added Catalogs (VACs). This paper concludes the release of SDSS-IV survey data. SDSS continues into its fifth phase with observations already underway for the Milky Way Mapper (MWM), Local Volume Mapper (LVM) and Black Hole Mapper (BHM) surveys.
202 citations
TL;DR: In this paper, the authors presented a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $|b|>40^{\circ}$ and $|Z|>2$ kpc.
Abstract: In the $\Lambda$CDM paradigm the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with $Gaia$, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $|b|>40^{\circ}$ and $|Z|>2$ kpc. We identify known structures including the high-$\alpha$ disk, the in-situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), $Gaia$-Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H]$=-0.5$), a low eccentricity structure that rises a surprising 10 kpc off the plane, (ii, iii) Arjuna ([Fe/H]$=-1.2$) and I'itoi ([Fe/H]$ $80$\%$ of the halo is built by two massive ($M_{\star}\sim10^{8}-10^{9}M_{\odot}$) accreted dwarfs: GSE ([Fe/H]$=-1.2$) within 25 kpc, and Sgr ([Fe/H]$=-1.0$) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H]$\approx-1.2$). We attribute $\gtrsim$95$\%$ of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.
184 citations
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