Molecular Gas and Star Formation in Nearby Disk Galaxies

TL;DR

This study examines the relationship between molecular gas and star formation in 30 nearby disk galaxies, revealing a roughly 2.2 Gyr depletion time and complex dependencies on galaxy properties and environmental factors.

Contribution

It provides the first detailed analysis of molecular gas and star formation correlations across a large galaxy sample, highlighting the importance of conversion factor variations.

Findings

Molecular gas depletion time is approximately 2.2 Gyr with 0.3 dex scatter.

The SFR surface density scales roughly linearly with molecular gas surface density.

Environmental factors like galaxy mass, metallicity, and dust-to-gas ratio influence depletion time.

Abstract

We compare molecular gas traced by 12CO(2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Sig_mol and Sig_SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, t_dep^mol = Sig_mol / Sig_SFR. At our 1 kpc common resolution, CO correlates closely with many tracers of the recent SFR. Weighting each line of sight equally and using a fixed, Milky Way alpha_CO, our data yield a molecular gas depletion time, t_dep^mol=Sig_mol/Sig_SFR ~ 2.2 Gyr with 0.3 dex scatter, in good agreement with literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density and find N=1+/-0.15 for our full data set with some variation from galaxy to galaxy. However, we caution that a power law treatment oversimplifies the topic given that we observe correlations between t_dep^mol and other local and global quantities. The strongest of these are a decreased t_dep^mol in low-mass, low-metallicity galaxies and a correlation of the kpc-scale t_dep^mol with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor that depends on D/G in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes it a conservative one. After applying a D/G-dependent alpha_CO, some weak correlations between t_dep^mol and local conditions persist. In particular, we observe lower t_dep^mol and enhanced CO excitation associated with some nuclear gas concentrations. These appear to reflect real enhancements in the SFR/H2 and t_dep appears multivalued at fixed Sig_mol, supporting the the idea of "disk" and "starburst" modes driven by environmental factors.