The Pristine survey X: a large population of low-metallicity stars
  permeates the Galactic disk
  The orbits of the least chemically enriched stars open a window on the
formation of our Galaxy when it was still in its infancy. The common picture is
that these low-metallicity stars are distributed as an isotropic,
pressure-supported component since these stars were either accreted from the
early building blocks of the assembling Milky Way, or were later brought by the
accretion of faint dwarf galaxies. Combining the metallicities and radial
velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and
proper motions for an unprecedented large and unbiased sample of very
metal-poor stars at $[Fe/H]\leq-2.5$ we show that this picture is incomplete.
This sample shows strong statistical evidence (at the $5.0\sigma$ level) of
asymmetry in their kinematics, favouring prograde motion. Moreover, we find
that $31\%$ of the stars that currently reside in the disk do not venture
outside of the disk plane throughout their orbit. The discovery of this
population implies that a significant fraction of stars with iron abundances
$[Fe/H]\leq-2.5$ formed within or concurrently with the Milky Way disk and that
the history of the disk was quiet enough to allow them to retain their
disk-like orbital properties.
