The variation in molecular gas depletion time among nearby galaxies: what are the main parameter dependences?

We re-analyze correlations between global molecular gas depletion time (tdep) and galaxy parameters including stellar mass, specific star formation rate, stellar surface density and concentration index. The analysis is based on the COLD GASS survey, which includes galaxies with stellar masses in the range 10 10 – 10 11.5 M ⊙ with molecular gas mass estimates derived from CO(J=1-0) line measurements. We improve on previous work by Saintonge et al (2011b) by estimating star formation rates using the combination of GALEX FUV and WISE 22 µm data and by deriving tdep within a fixed aperture set by the IRAM beam size. In our new study we find correlations with much smaller scatter. Dependences of the depletion time on galaxy structural parameters such as stellar surface density and concentration index are now weak or absent. Differences with previous work arise because dust extinction as measured by the ratio of 22 micron to far-UV flux in the galaxy, correlates strongly with galaxy structural parameters. We further demonstrate that the primary global parameter correlation is between tdep and sSFR; all other remaining correlations can be shown to be induced by this primary dependence. This implies that galaxies with high current-topast-averaged star formation activity, will drain their molecular gas reservoir sooner. We then analyze molecular gas depletion times on 1-kpc scales in galactic disks using data from the HERACLES survey. There is remarkably good agreement between the global tdep versus sSFR relation for the COLD GASS galaxies and that derived for 1 kpc scale grid regions in disks. The strong correlation between tdep and sSFR extends continuously over a factor of 10 in tdep from log SFR/M� = −11.5 to −9, i.e. from nearly quiescent patches of the disk to disk regions with very strong star formation. This leads to the conclusion that the local molecular gas depletion time in galactic disks is dependent on the local fraction of young-to-old stars.