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 \sim 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 \sim 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 < \bar { v } _ { \theta } ( \mathrm { kms } ^ { -1 } ) < 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 M _ { vir } > 10 ^ { 10 } M _ { \odot } 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 .