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 $1\times {10}^8<{\mathrm{M}}_{\star }<2\times {10}^{10}[{\mathrm{M}}_{\odot }]$, a virial mass range between $3\times {10}^{10}<{\mathrm{M}}_{\star }<6\times {10}^{11}[{\mathrm{M}}_{\odot }]$, and be as young as $8\lesssim \mathrm{A}\mathrm{g}\mathrm{e}\lesssim 12.8$ [Gyr] ($1\lesssim z\lesssim 6$); ii) are predominantly centrally concentrated, with $\sim 50\mathrm{\%}$ 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., $0.25\lesssim ⟨\kappa \mathrm{/}{\kappa }_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}⟩\lesssim 0.8$); iv) present [$\alpha$/Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host’s old disc; v) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that ~60% of the proto-Milky Way galaxies are comprised by 1 dominant system ($1\mathrm{/}5\lesssim$M${}_{\star }$/M${}_{\star ,\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{o}-\mathrm{M}\mathrm{i}\mathrm{l}\mathrm{k}\mathrm{y}\mathrm{W}\mathrm{a}\mathrm{y}}$$\lesssim 4\mathrm{/}5$) and 4-5 lower mass systems (M${}_{\star }$/M${}_{\star ,\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{o}-\mathrm{M}\mathrm{i}\mathrm{l}\mathrm{k}\mathrm{y}\mathrm{W}\mathrm{a}\mathrm{y}}$$\lesssim 1\mathrm{/}10$); the other ~40% 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.