  We studied the effect of interstellar gas conditions on global galaxy
simulations by considering three different models for the ISM. Our first model
included only radiative cooling down to 300 K, our second model added an
additional background heating term due to photoelectric heating, and our third
model uses an isothermal equation of state with a temperature of 10^4 K and no
explicit heating or cooling. Two common prescriptions for star formation are
implemented in each case. The first is based on cosmological simulations with a
low threshold for star formation but also a low efficiency. The second assumes
stars form only in high density regions but with a higher efficiency. We also
explore the effects of including feedback from type II supernovae. We find that
the different ISM types produce marked differences in the structure of the disk
and temperature phases present in the gas, although inclusion of feedback
largely dominates these effects. In particular, size of the star-forming clumps
was increased both by background heating and by enforcing an isothermal ISM. We
also looked at the one dimensional profiles and found that a lognormal PDF
provides a good fit for all our simulations over several orders of magnitude in
density. Overall, despite noticeable structural differences, the star formation
properties in the disk are largely insensitive to ISM type and agree reasonably
well with observations.
