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

Abstract

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 (2010) 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 starformation 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 which 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 is yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may be in fact fossil records of its last significant merger.

The tale of the tail – disentangling the high transverse velocity stars in Gaia DR2

“…Zolotov et al 2009;Brook et al 2012). Our findings are similar to those of Haywood et al (2018). They pointed out that the RS is likely to be made up mostly of thick disc stars and that the BS is likely coming from an accreted population, due to its [α/Fe] abundances.…”

“…• Our interpretation of the two sequences is in alignment with Haywood et al (2018) and Gallart et al (2019), namely the BS is mostly composed of accreted stars, whereas the RS is mostly composed of heated thick-disc material;…”

“…APPENDIX B: APO − Z M AX PLANE As mentioned in Section 5, Haywood et al (2018) investigated the distribution of high v t stars in the R apo − z max plane. They noticed that the stars followed tracks in this plane and attributed this to the effects of accretion.…”

Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way

“…This fit with the previously held idea that the GES stars were younger than the main progenitor halo stars. Haywood et al (2018) invoked the difference in star formation efficiency for explaining the two tracks in the [Fe/H] vs [O/Fe] plane, without putting constraints on the differences in the duration of star formation. Fernández-Alvar et al (2019) suggest that it is a combination of both differences in the duration and the intensity of star formation, combined with differences in the initial mass functions (see also Kobayashi et al 2014) of the two merging galaxies, that explains the different chemical evolution tracks.…”

The building blocks of the Milky Way halo using APOGEE and Gaia or Is the Galaxy a typical galaxy?

“…An exciting recent result in GA was the discovery by different groups that the local stellar halo is dominated by the remnants of the accretion of a single satellite galaxy (Haywood et al 2018;Helmi et al 2018;Belokurov et al 2018;Myeong et al 2018;Mackereth et al 2019). The Gaia-Enceladus/Sausage (GE/S) system is thought to be a dwarf galaxy which merged with the MW ∼ 10 Gyr ago.…”