Applying a star formation model calibrated on high-resolution interstellar medium simulations to cosmological simulations of galaxy formation

Modern high-resolution simulations of the interstellar medium (ISM) have shown that key factors in governing star formation are the competing influences of radiative dissipation, pressure support driven by stellar feedback, and gravity. However, cosmological simulations of galaxy formation are unable to resolve this physics in detail and therefore rely on approximate treatments, often taking the form of empirical subgrid models of the ISM expressed in terms of an effective equation of state (EOS). Here we seek to improve these heuristic models by directly fitting to results of the high-resolution TIGRESS simulations, which have shown that the dynamical equilibrium of the ISM can be understood in terms of a pressure-regulated feedback-modulated (PRFM) model for star formation. We explore a simple subgrid model that draws on the PRFM concept but uses only local quantities. It accurately reproduces the TIGRESS scalings for pure gas discs, but predicts slightly less star formation in the presence of an additional stellar disc. We compare this to the older Springel & Hernquist and TNG prescriptions by applying all three models to isolated simulations of disc galaxies as well as to a set of high-resolution zoom-in simulations carried out with a novel 'multizoom' technique that we introduce here. The softer TIGRESS EOS produces substantially thinner disc galaxies, which has important ramifications for disc stability and galaxy morphology. The stellar mass of galaxies is hardly modified at low redshift, reflecting the dominating influence of large-scale gaseous inflows and outflows, which are not sensitive to the EOS itself.