Kavli Affiliate: Lina Necib
| First 5 Authors: Danny Horta, Emily C. Cunningham, Robyn Sanderson, Kathryn V. Johnston, Alis Deason
| Summary:
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}}ranglelesssim0.8$);
$iii$) 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.
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