Co-formation of the disc and the stellar halo★

Using a large sample of main sequence stars with 7D 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 with the tangential one) is mildly radial, with 0.2 -1.7, however, we measure extreme values of 0.9. Across the metallicity range considered, namely -3 10 10 M around the epoch of the Galactic disc formation, between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialization of the massive progenitor's orbit, amplified by the action of the growing disc.