Physical properties of simulated galaxy populations at z=2 - II. Effects
  of cosmology, reionization and ISM physics
  We use hydrodynamical simulations from the OWLS project to investigate the
dependence of the physical properties of galaxy populations at redshift 2 on
the assumed star formation law, the equation of state imposed on the unresolved
interstellar medium, the stellar initial mass function, the reionization
history, and the assumed cosmology. This work complements that of Paper I,
where we studied the effects of varying models for galactic winds driven by
star formation and AGN. The normalisation of the matter power spectrum strongly
affects the galaxy mass function, but has a relatively small effect on the
physical properties of galaxies residing in haloes of a fixed mass.
Reionization suppresses the stellar masses and gas fractions of low-mass
galaxies, but by z = 2 the results are insensitive to the timing of
reionization. The stellar initial mass function mainly determines the physical
properties of galaxies through its effect on the efficiency of the feedback,
while changes in the recycled mass and metal fractions play a smaller role. If
we use a recipe for star formation that reproduces the observed star formation
law independently of the assumed equation of state of the unresolved ISM, then
the latter is unimportant. The star formation law, i.e. the gas consumption
time scale as a function of surface density, determines the mass of dense,
star-forming gas in galaxies, but affects neither the star formation rate nor
the stellar mass. This can be understood in terms of self-regulation: the gas
fraction adjusts until the outflow rate balances the inflow rate.
