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 × 108 < M· < 2 × 1010 [MS], a virial mass range between 3 × 1010 < M· < 6 × 1011 [MS], and be as young as 8 Age 12.8 [Gyr] (1 z 6); (ii) are pre-dominantly centrally concentrated, with 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 κ/κdisc) 0.8); (iv) present [α/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 of the proto-Milky Way galaxies are comprised by 1 dominant system (1/5M·M proto-MilkyWay 4/5) and 4-5 lower mass systems (M·M, proto-MilkyWay1/10); the other are comprised by 2 dominant systems and 3-4 lower mass systems. These massive/dominant proto-Milky 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.