Molecular Hydrogen and Global Star Formation Relations in Galaxies

TL;DR

This study uses hydrodynamical simulations to explore how molecular hydrogen influences star formation relations in galaxies, revealing consistent local relations and explaining deviations in global star formation laws.

Contribution

It introduces a detailed ISM model linking star formation to molecular gas and explains observed variations in star formation relations across different galaxy sizes.

Findings

The molecular-gas surface density follows a Schmidt-Kennicutt relation with index ~1.4.

The total gas surface density-star formation rate relation steepens in smaller galaxies.

Deviations from the global relation are mainly due to spatial variations in molecular fraction.

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<10^4K) 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 Sigma_SFR \propto Sigma_Hmol^n_mol with n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to spatial trends in the molecular fraction f_H2 and may explain observed deviations from the global Schmidt-Kennicutt relation.