The Variation in Molecular Gas Depletion Time among Nearby Galaxies:
  What are the Main Parameter Dependencies?
  We re-analyze correlations between global molecular gas depletion time (Tdep)
and galaxy parameters including stellar mass, specific star formation rate
(sSFR), stellar mass surface density and concentration index. The analysis is
based on the COLD GASS survey, which includes galaxies with stellar mass in the
range 10^10 - 10^11.5 Msun with molecular gas mass estimates derived from
CO(1-0) line measurements. We improve on previous work by Saintonge et al.
(2011b) by estimating SFRs using the combination of GALEX FUV and WISE 22
micron 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 Tdep on galaxy structural parameters such as stellar mass
surface density and concentration index are now weak or absent. Differences
with previous work arise because dust extinction 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-to-past-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.
