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Previous studies based on the analysis of Gaia DR2 data have revealed thataccreted stars, possibly originating from a single progenitor satellite, are asignificant component of the halo of our Galaxy, potentially constituting mostof the halo stars at $\rm [Fe/H] < -1$ within a few kpc from the Sun andbeyond. In this paper, we couple astrometric data from Gaia DR2 with elementalabundances from APOGEE DR14 to characterize the kinematics and chemistry ofin-situ and accreted populations up to $\rm [Fe/H] \sim -2$. Accreted starsappear to significantly impact the Galactic chemo-kinematic relations, not onlyat $\rm [Fe/H] < -1$, but also at metallicities typical of the thick andmetal-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 findthat the stellar kinematic fossil record shows the imprint left by thisaccretion event which heated the old Galactic disc. We are able to age-datethis kinematic imprint, showing that the accretion occurred between 9 and 11Gyr 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 iscomposed of metal-rich thick disc stars. Moreover, we suggest that thismetal-rich thick disc component dominates the stellar halo of the inner Galaxy.The new picture that emerges from this study is one where the standardnon-rotating in-situ halo population, the collapsed halo, seems to be moreelusive than ever.

astronomy and astrophysics

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