We analyse a set of very metal-poor stars for which accurate chemical abundances have been obtained as part of the ESO Large Program “ First stars ” in the light of the Gaia DR2 data .
The kinematics and orbital properties of the stars in the sample show they probably belong to the thick disc , partially heated to halo kinematics , and to the accreted Nissen & Schuster-Gaia Sausage-Enceladus satellite .
The continuity of these properties with stars at both higher ( [ Fe / H ] > -2 ) and lower metallicities ( [ Fe / H ] < -4. ) suggests that the Galaxy at [ Fe / H ] \lesssim - 0.5 and down to at least [ Fe / H ] \sim - 6 is dominated by these two populations .
In particular , we show that the disc extends continuously from [ Fe / H ] \leq - 4 ( where stars with disc-like kinematics have been recently discovered ) up to [ Fe / H ] \geq - 2 , the metallicity regime of the Galactic thick disc .
There exists indeed an “ ultra-metal poor thick disc ” , which constitutes the extremely metal-poor tail of the canonical Galactic thick disc , and which extends this latter from [ Fe / H ] \sim - 0.5 up to the most metal-poor stars discovered in the Galaxy up to date .
These results suggest that the disc may be the main , and possibly the only stellar population that has formed in the Galaxy at these metallicities .
This requires that the dissipative collapse that led to the formation of the old Galactic disc must have been extremely fast .
We also discuss these results in the light of recent simulation efforts made to reproduce the first stages of Milky Way-type galaxies .