Tracing the formation of the Milky Way through ultra metal-poor stars

We use Gaia DR2 astrometric and photometric data, combined with MESA isochrone and luminosity function models and radial velocities from literature to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ($\FeH 11\Gyr$) and that they are expected to belong to the Milky Way halo, we find that these 41 stars (18 dwarf stars and 23 giants or sub-giants) are currently within $\sim20$ kpc from the Sun and that they map a wide variety of orbits. A large fraction of those stars remains confined to the inner parts of the halo and was likely formed or accreted early-on in the history of the Milky Way, while others have larger apocentres ($>30\kpc$), hinting at later accretion from dwarf galaxies. Of particular interest, we find evidence that a significant fraction of all known UMP stars ($\sim29$\%) are on prograde orbits confined within $3\kpc$ of the Milky Way plane ($J_z < 100 \kms \kpc$). One intriguing interpretation is that these stars belonged to the massive building block(s) of the proto-Milky Way that formed the backbone of the Milky Way disc. Alternatively, they may have been brought into the Milky Way by one or more accretion events whose orbit was dragged into the plane by dynamical friction before disruption. The combination of the exquisite Gaia DR2 data and surveys of the very metal-poor sky opens an exciting era in which we can trace the very early formation of the Milky Way.