Co-formation of the Galactic disc and the stellar halo
  Using a large sample of Main Sequence stars with 7-D measurements supplied by
Gaia and SDSS, we study the kinematic properties of the local (within ~10 kpc
from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid
evolves strongly with metallicity. At the low [Fe/H] end, the orbital
anisotropy (the amount of motion in the radial direction compared to the
tangential one) is mildly radial with 0.2<beta<0.4. However, for stars with
[Fe/H]>-1.7 we measure extreme values of beta~0.9. Across the metallicity range
considered, i.e. -3<[Fe/H]<-1, the stellar halo's spin is minimal, at the level
of 20<v_theta (km/s) <30. Using a suite of cosmological zoom-in simulations of
halo formation, we deduce that the observed acute anisotropy is inconsistent
with the continuous accretion of dwarf satellites. Instead, we argue, the
stellar debris in the inner halo were deposited in a major accretion event by a
satellite with Mvir>10^10 Msun around the epoch of the Galactic disc formation,
i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the
dramatic radialisation of the massive progenitor's orbit, amplified by the
action of the growing disc.
