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

Danny Horta, Emily C. Cunningham, Robyn Sanderson, Kathryn V. Johnston, Alis Deason, Andrew Wetzel, Fiona McCluskey, Nicolás Garavito-Camargo, Lina Necib, Claude-André Faucher-Giguère, Arpit Arora, Pratik J. Gandhi

Published 2023-07-28Version 1

Observational studies are finding stars believed to be relics of the earliest stages of hierarchical mass assembly of the 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: $i$) are predominantly centrally concentrated, with $\sim50\%$ of the stars contained within $5-10$ kpc; $ii$) on average show weak but systematic net rotation in the plane of the host's disc at $z=0$ (i.e., 0.25$\lesssim$$\langle$$\kappa$/$\kappa_{\mathrm{disc}}\rangle\lesssim0.8$); $iii$) present [$\alpha$/Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host's old disc; $iv$) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that $\sim$60$\%$ of the proto-Milky Way galaxies are comprised by 1 dominant system (1/5$\lesssim$M$_{\star}$/M$_{\star,\mathrm{proto-Milky Way}}$$\lesssim$4/5) and $4-5$ lower mass systems (M$_{\star}$/M$_{\star,\mathrm{proto-Milky Way}}$$\lesssim$1/10); the other $\sim$40$\%$ 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 suggest that large/rich chemical-kinematic-age samples of metal-poor stars in the inner Galaxy should help characterise the different mass fragments of the proto-Milky Way. These data may also help reveal if the Milky Way formed from one or two dominant systems.