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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
Open accessAbstract
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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“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
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“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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Metadata
- 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)
- Venue
- Astronomy & Astrophysics
- Publisher
- EDP Sciences
- Published
- 2019-11-21
- Volume
- 632
- Pages
- A4
- ISSN
- 0004-6361 (ISSN-L)
- 1432-0746
- 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
- hybrid
- doi.org
- 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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