In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2

By M. Haywood, Paola Di Matteo, M. Lehnert, O. Snaith, S. Khoperskov, Ana G'omez. Astrophysical Journal, 2018

199 citations recorded by the source corpus.

We investigate the nature of the double color–magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically hot tail of the thick disk population. Information from Nissen & Schuster and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low star formation efficiency during its formation. In dynamical and orbital spaces, such as the “Toomre diagram,” the two sequences show a significant overlap, but with a tendency for stars on the blue-color sequence to dominate regions with no or retrograde rotation and high total orbital energy. In the plane defined by the maximal vertical excursion of the orbits versus their apocenters, stars of both sequences redistribute into discrete wedges. We conclude that stars that are typically assigned to the halo in the solar vicinity are actually both accreted stars lying along the blue sequence in the HR diagram, and the low rotational velocity tail of the old Galactic disk, possibly dynamically heated by past accretion events. Our results imply that a halo population formed in situ and responsible for the early chemical enrichment prior to the formation of the thick disk has yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may in fact be fossil records of its last significant merger.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/9629c0c7ba694dedf4802d8f680e57455e94b58e

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Citations in the corpus

The biggest splash (2019, methodology): “(i) In terms of metallicity, the Splash can be seen most easily between [Fe/H] = −0.7 and −0.2 (as already seen in Bonaca et al. 2017; Haywood et al. 2018; Di Matteo et al. 2018; Gallart et al. 2019).”
The Sausage Globular Clusters (2018, result): “Although identified in the SDSS-Gaia catalogue, recent investigations by Haywood et al. (2018) with the new Gaia Data Release 2 (DR2) catalogue (Gaia Collaboration et al. 2018) support the original hypothesis.”
Evidence for two early accretion events that built the Milky Way stellar halo (2019, background): “…Gaia has provided compelling evidence for the nearly head-on collision of a Magellanic-sized dwarf galaxy with the nascent Milky Way some 8 to 10 billion years ago, the so-called ‘Gaia Sausage’ (see e.g. Belokurov et al. 2018; Myeong et al. 2018c,d; Haywood et al. 2018; Fattahi et al. 2019).”