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
  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. 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 9 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.
