The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback

We consider the effects of different criteria for determining where stars will form in gas on galactic scales, in simulations with high (1 pc) resolution, with explicitly resolved physics of GMC formation and destruction and stellar feedback from supernovae, radiation pressure, stellar winds, and photo-heating. We compare: (1) a self-gravity criterion (based on the local virial parameter and the assumption that self-gravitating gas collapses to high density in a single free-fall time), (2) a fixed density threshold, (3) a molecular-gas law, (4) a temperature threshold, (5) a requirement that the gas be Jeans-unstable, (6) a criteria that cooling times be shorter than dynamical times, and (7) a convergent-flow criterion. We consider all of these in both a MW-like and high-density (starburst or high-redshift) galaxy. With feedback present, all models produce identical integrated star formation rates (SFRs), in good agreement with the Kennicutt relation; without feedback all produce orders-of-magnitude excessive SFRs. This is totally dependent on feedback and independent of the SF law, even if the “local ” collapse efficiency is 100%. However, the predicted spatial and density distribution depend strongly on the SF criteria. Because cooling rates are generally fast within galaxy disks, and gas is turbulent, criteria (4)-(7) are very “weak” and spread the SF uniformly over most of the disk (down to densities n ∼ 0.01−0.1cm−3). A molecula