Molecular Hydrogen and Global Star Formation Relations in Galaxies

Abstract

Weusehydrodynamicalsimulationsof diskgalaxiestostudyrelationsbetweenstarformationandpropertiesof the molecular ISM. We implement a model for the ISM that includes low-temperature (T < 10 4 K) cooling, directly ties the SFR 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 formSFR / � n mol H2 , with nmol � 1:4 independent of galaxy mass, and a total gas surface densityYSFR relationSFR / � n tot gas with a power-law index that steepens from ntot � 2 for large galaxies to ntotk4 for small dwarf galaxies. We show that deviations from the disk-averaged � SFR / � 1:4 gas correlation determined by Kennicutt owe primarily to spatial trends in the molecular fractionfH2 and may explain observed deviations from the global Schmidt-Kennicutt relation. In our model, such deviations occur in regionsof theISMwhere thefraction of gasmassinmolecularformisdeclining orsignificantlylessthanunity.Long gas consumption timescales in low-mass and low surface brightness galaxies may owe to their small fractions of molecular gas rather than mediation by strong supernova-driven winds. Our simulations also reproduce the observed relations between ISM pressure and molecular fraction and between SFR, 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 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 pdf's corresponding to individual, approximately isothermal phases. We also present a simple method to mitigate numerical Jeansfragmentation of dense,cold gas in SPH codesthrough the adoption of a density-dependent pressure floor. Subject headingg Galaxy: evolution — Galaxy: formation — Galaxy: structure — ISM: general