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

We re-analyse correlations between global molecular gas depletion time (t(dep)) and galaxy parameters for nearby galaxies from the COLD GASS survey. We improve on previous work of Saintonge et al. by estimating star formation rates using the combination of Galaxy Evolution Explorer far-ultraviolet and Wide-field Infrared Survey Explorer 22 mu m data and by deriving t(dep) within a fixed aperture set by the beam size of gas observation. 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. We demonstrate that the primary global parameter correlation is between t(dep) and specific star formation rate (sSFR); all other remaining correlations can be shown to be induced by this primary dependence. This implies that galaxies with high current-to-past-averaged star formation activity, will drain their molecular gas reservoir sooner. We then analyse t(dep) on 1 kpc scales in galactic discs using data from the HERA CO-Line Extragalactic Survey survey. There is remarkably good agreement between the global t(dep)-sSFR relation for the COLD GASS galaxies and that derived for 1 kpc scale grids in discs. This leads to the conclusion that the local molecular gas depletion time in galactic discs is dependent on the local fraction of young-to-old stars.