UniverseMachine: Predicting Galaxy Star Formation over Seven Decades of
  Halo Mass with Zoom-in Simulations
  We apply the empirical galaxy--halo connection model UniverseMachine to dark
matter-only zoom-in simulations of isolated Milky Way (MW)--mass halos along
with their parent cosmological simulations. This application extends
\textsc{UniverseMachine} predictions into the ultra-faint dwarf galaxy regime
($ 10^{2}\,\mathrm{M_{\odot}} \leqslant M_{\ast} \leqslant
10^{5}\,\mathrm{M_{\odot}}$) and yields a well-resolved stellar mass--halo mass
(SMHM) relation over the peak halo mass range $10^8\,\mathrm{M_{\odot}}$ to
$10^{15}\,\mathrm{M_{\odot}}$. The extensive dynamic range provided by the
zoom-in simulations allows us to assess specific aspects of dwarf galaxy
evolution predicted by \textsc{UniverseMachine}. In particular, although
UniverseMachine is not constrained for dwarf galaxies with $M_* \lesssim
10^{8}\,\mathrm{M_{\odot}}$, our predicted SMHM relation is consistent with
that inferred for MW satellite galaxies at $z=0$ using abundance matching.
However, UniverseMachine predicts that nearly all galaxies are actively star
forming below $M_{\ast}\sim 10^{7}\,\mathrm{M_{\odot}}$ and that these systems
typically form more than half of their stars at $z\lesssim 4$, which is
discrepant with the star formation histories of Local Group dwarf galaxies that
favor early quenching. This indicates that the current UniverseMachine model
does not fully capture galaxy quenching physics at the low-mass end. We
highlight specific improvements necessary to incorporate environmental and
reionization-driven quenching for dwarf galaxies, and provide a new tool to
connect dark matter accretion to star formation over the full dynamic range
that hosts galaxies.
