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

Understand

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 $\rm [Fe/H] < -1$ 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 characterize the kinematics and chemistry of in-situ and accreted populations up to $\rm [Fe/H] \sim -2$.
• Accreted stars appear to significantly impact the Galactic chemo-kinematic relations, not only at $\rm [Fe/H] < -1$, but also at metallicities typical of the thick and metal-poor thin discs.
• They constitute about 60% of all stars at $\rm [Fe/H] < -1$, the remaining 40% being made of (metal-weak) thick disc stars.