2023/07/28 by Horta, Danny, Cunningham, Emily C., Sanderson, Robyn +9 · 2 citations
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2307.15741
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[M\odot], a virial mass range between 3×1010<M⋆<6×1011[M\odot], and be as young as 8 \lesssim Age \lesssim 12.8 [Gyr] (1\lesssim z \lesssim 6); ii) are predominantly centrally concentrated, with ∼50% 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⟨κ/κdisc⟩\lesssim0.8); iv) present [α/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 (1/5\lesssimM⋆/M⋆,proto-Milky Way\lesssim4/5) and 4-5 lower mass systems (M⋆/M⋆,proto-Milky Way\lesssim1/10); 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.