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 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
$1times10^{8}<mathrm{M}_{star}<2times10^{10}[mathrm{M}_{odot}]$, a virial
mass range between
$3times10^{10}<mathrm{M}_{star}<6times10^{11}[mathrm{M}_{odot}]$, and be
as young as $8 lesssim mathrm{Age} lesssim 12.8$ [Gyr] ($1lesssim z
lesssim 6$); ii) are predominantly centrally concentrated, with $sim50%$ 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.,
$0.25lesssimlanglekappa/kappa_{mathrm{disc}}ranglelesssim0.8$); 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
($1/5lesssim$M$_{star}$/M$_{star,mathrm{proto-Milky Way}}$$lesssim4/5$)
and 4-5 lower mass systems (M$_{star}$/M$_{star,mathrm{proto-Milky
Way}}$$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.
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