Molecular Hydrogen and Global Star Formation Relations in Galaxies

We use hydrodynamical simulations of disk galaxies to study relations between star formation and properties of the molecular interstellar medium (ISM). We implement a model for the ISM that includes low-temperature (T < 104 K) cooling, directly ties the star formation rate to the molecular gas density, and accounts for the destruction of H2 by an interstellar radiation field from young stars. We demonstrate that the ISM and star formation model simultaneously produces a spatially-resolved molecular-gas surface density Schmidt-Kennicutt relation of the form ΣSFR ∝ Σmol H2 with nmol ≈ 1.4 independent of galaxy mass, and a total gas surface density – star formation rate relation ΣSFR ∝ Σntot gas with a power-law index that steepens from ntot ∼ 2 for large galaxies to ntot & 4 for small dwarf galaxies. We show that deviations from the disk-averaged ΣSFR ∝ Σ1.4 gas correlation determined by Kennicutt (1998) owe primarily to spatial trends in the molecular fraction fH2 and may explain observed deviations from the global Schmidt-Kennicutt relation. In our model, such deviations occur in regions of the ISM where the fraction of gas mass in molecular form is declining or significantly less than unity. Long gas consumption time scales in low-mass and low surface brightness galaxies may owe to their small fractions of molecular gas rather than mediation by strong supernovae-driven winds. Our simulations also reproduce the observed relations between ISM pressure and molecular fraction and between star formation rate, gas surface density, and disk angular frequency. We show that the Toomre criterion that accounts for both gas and stellar densities correctly predicts the onset of star formation in our simulated disks. We examine the density and temperature distributions of the ISM in simulated galaxies and show that the density probability distribution function (PDF) generally exhibits a complicated structure with multiple peaks corresponding to different temperature phases of the gas. The overall density PDF can be well-modeled as a sum of lognormal PDFs corresponding to individual, approximately isothermal phases. We also present a simple method to mitigate numerical Jeans fragmentation of dense, cold gas in Smoothed Particle Hydrodynamics codes through the adoption of a density-dependent pressure floor. Subject headings: Galaxies, Star Formation