A parametric study of possible solutions to the high-redshift
  overproduction of stars in modeled dwarf galaxies
  Both numerical hydrodynamic and semi-analytic cosmological models of galaxy
formation struggle to match observed star formation histories of galaxies in
low mass halos (M$_{\rm{H}} \lesssim 10^{11} M_\odot$), predicting more star
formation at high redshift and less star formation at low redshift than
observed. The fundamental problem is that galaxies' gas accretion and star
formation rates are too closely coupled in the models: the accretion rate
largely drives the star formation rate. Observations point to gas accretion
rates that outpace star formation at high redshift, resulting in a buildup of
gas and a delay in star formation until lower redshifts. We present three
empirical adjustments of standard recipes in a semi-analytic model motivated by
three physical scenarios that could cause this decoupling: 1) the mass-loading
factors of outflows driven by stellar feedback may have a steeper dependence on
halo mass at earlier times, 2) the efficiency of star formation may be lower in
low mass halos at high redshift, and 3) gas may not be able to accrete
efficiently onto the disk in low mass halos at high redshift. These new
recipes, once tuned, better reproduce the evolution of $f_\star \equiv
M_\star/M_{\rm{H}}$ as a function of halo mass as derived from abundance
matching over redshifts $z=0$ to $3$, though they have different effects on
cold gas fractions, star formation rates, and metallicities. Changes to gas
accretion and stellar-driven winds are promising, while direct modification of
the star formation timescale requires drastic measures that are not physically
well-motivated.
