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Cited by 223Stellar, planetary, and galactic studiesGalaxies: Formation, Evolution, PhenomenaAstrophysics and Star Formation Studies

The Milky Way has no in-situ halo other than the heated thick disc: Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE

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Di Matteo, P., Haywood, M., Lehnert, M. D., Katz, D., Khoperskov, S., Snaith, O. N., Gómez, A., Robichon, N.·2019·Astronomy & Astrophysics
DOI 10.1051/0004-6361/201834929
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Abstract

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at [Fe/H] < −1 within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterise the kinematics and chemistry of in-situ and accreted populations up to [Fe/H] ∼ −2. Accreted stars appear to significantly impact the galactic chemo–kinematic relations, not only at [Fe/H] < −1, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at [Fe/H] < −1, the remaining 40% being made of (metal-weak) thick-disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event, which heated the old galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between nine and 11 Gyr ago, and that it led to the last significant heating of the galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the galactic halo. Indeed, about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick-disc stars. Moreover, we suggest that this metal-rich thick-disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard, non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

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  • The second sequence in the color-magnitude diagram is populated by stars that are kinematically part of the tail of the thick disk.

    “However, the second sequence is populated by stars that, kinematically, are clearly part of the tail of the thick disk seen in Figure 5b (Di Matteo et al. 2019, Haywood et al. 2018).”

    From Streams, Substructures, and the Early History of the Milky Way (2020) · cites this paper as Di Matteo et al. 2019

    The Introduction describes a red sequence in the Gaia color–magnitude diagram but says its overlap with accreted stars is significant. Section 4.1 finds that heated thick-disc stars make up much of the kinematically defined halo near the Sun. The paper therefore supports a thick-disc contribution, but not the categorical claim that stars in the second color–magnitude sequence are clearly part of the thick-disc tail. (LLM, conf=0.82)

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  • The proto-thick disk stars were likely dynamically heated during the merger with Gaia-Enceladus.

    “These (proto-)thick disk stars were likely dynamically heated during the merger with Gaia-Enceladus (Helmi et al. 2018, Di Matteo et al. 2019).”

    From Streams, Substructures, and the Early History of the Milky Way (2020) · cites this paper as Di Matteo et al. 2019

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Authors
Di Matteo, P., Haywood, M., Lehnert, M. D., Katz, D., Khoperskov, S., Snaith, O. N., Gómez, A., & Robichon, N.
Affiliations
  • Centre National de la Recherche Scientifique (FR)
  • Korea Institute for Advanced Study (KR)
ORCID
Venue
Astronomy & Astrophysics
Publisher
EDP Sciences
Published
2019-11-21
Volume
632
Pages
A4
ISSN
  • 0004-6361 (ISSN-L)
  • 1432-0746
DOI
10.1051/0004-6361/201834929
OpenAlex
W3103964408
Cited by
222
References
123
Citations by year
2026: 13 · 2025: 30 · 2024: 30 · 2023: 34 · 2022: 27 · 2021: 49 · 2020: 35 · 2019: 4
FWCI
15.48
Type
journal article
Language
en
Open access
Licence
Locations
  • Astronomy and Astrophysics (publishedVersion, cc-by)
  • arXiv (Cornell University) (submittedVersion)
  • Springer Link (Chiba Institute of Technology) (submittedVersion, cc-by)
Funders
  • Smithsonian Institution
  • U.S. Department of Energy
  • Alfred P. Sloan Foundation
  • Carnegie Institution of Washington
  • University of Utah
  • Johns Hopkins University
  • Carnegie Mellon University
  • Carnegie Institution for Science
  • European Space Agency
  • Agence Nationale de la Recherche (ANR-15-CE31-0007)
  • Russian Science Foundation (19–72–20089)
  • Instituto de Astrofísica de Canarias
  • Office of Science
  • Smithsonian Astrophysical Observatory
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Stellar, planetary, and galactic studies
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysics and Star Formation Studies
Keywords
Milky Way stellar halo, thick disk, stellar kinematics, satellite accretion, disk heating, Gaia DR2

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