The Effect of the Interstellar Model on Star Formation Properties in Galactic Disks

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.