Molecular Hydrogen and Global Star Formation Relations in Galaxies
Abstract
(ABRIDGED) 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<104K) 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 SigmaSFR SigmaHmolnmol with nmol~1.4 independent of galaxy mass, and a total gas surface density -- star formation rate relation SigmaSFR Sigmagasntot 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 SigmaSFR Sigmagas1.4 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.