The proto-galaxy of Milky Way-mass haloes in the FIRE simulations

Observational studies are finding stars believed to be relics of the earliest stages of hierarchical mass assembly of the Milky Way (i.e. proto-galaxy). In this work, we contextualize these findings by studying the masses, ages, spatial distributions, morphology, kinematics, and chemical compositions of proto-galaxy populations from the 13 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. Our findings indicate that proto-Milky Way populations: (i) can have a stellar mass range between 1 x 10( 8) < M (*) < 2 x 10( 10) [Me], a virial mass range between 3 x 10( 10) < M (*) < 6 x 10(11) [Me], and be as young as 8 less than or similar to Age less than or similar to 12.8 [Gyr] (1 < z < 6); (ii) are pre-dominantly centrally concentrated, with similar to 50 per cent of the stars contained within 5-10 kpc; (iii) on average show weak but systematic net rotation in the plane of the host's disc at z = 0 (i.e. 0.25 < (K/K-disc) < 0.8); (iv) present [ alpha /Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host's old disc; and (v) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that similar to 60 per cent of the proto-Milky Way galaxies are comprised by 1 dominant system (1/5 less than or similar to M- * /M- * , (proto-MilkyWay )less than or similar to 4/5) and 4-5 lower mass systems (M (*) /M- * , (proto-MilkyWay) less than or similar to 1/10); the other similar to 40 per cent are comprised by 2 dominant systems and 3-4 lower mass systems. These massiv e/dominant proto-Milk y Way fragments can be distinguished from the lower mass ones in chemical-kinematic samples, but appear (qualitatively) indistinguishable from one another. Our results could help observational studies disentangle if the Milky Way formed from one or two dominant systems.