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

astronomy and astrophysics

Reviving old controversies: is the early Galaxy flat or round?  Investigations into the early phases of the Milky Way formation through  stellar kinematics and chemical abundances