Kavli Affiliate: Andrey Kravtsov
| First 5 Authors: Vance Wheeler, Vance Wheeler, , ,
| Summary:
We use intergalactic medium (IGM) metallicity distributions from several
state-of-the-art cosmological simulations of Milky Way analogs and a
semi-analytic model of ultra-faint dwarf galaxy (UFD) formation to model the
stellar metallicities of UFDs in MW-like environments. We study simulations
with different treatments of star formation, stellar feedback, and Population
III enrichment, and in all cases, we find that only a few percent of the IGM
accretable by UFD progenitors is enriched to metallicities $rm [Fe/H]ge-4$.
When the metallicity of accreted IGM in the semi-analytic galaxy formation
model is set using these IGM metallicity distributions, the model underpredicts
UFD metallicities and their scatter compared to the observed
luminosity–metallicity relation. Our results indicate that IGM enrichment is
not the dominant mechanism setting metallicities of UFD stars. Instead, UFD
stellar metallicity is determined primarily by the interplay between internal
enrichment and metal loss through feedback-driven outflows. We examine models
with different values of the maximum outflow mass loading factor $eta_rm
max$ and show that the full range of average stellar metallicities of UFDs at
$M_V<-7$ can be reproduced if the maximum mass loading factor varies in the
range $200lesssimeta_rm maxlesssim 2000$. We also consider stellar
metallicity distribution functions (MDFs) within individual model galaxies with
different assumptions about IGM enrichment and $eta_rm max$. We find that
all considered models are in reasonable agreement with observed UFD MDFs, with
model differences less than the uncertainties of current metallicity
measurements.
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