The variation in molecular gas depletion time among nearby galaxies: II
  the impact of galaxy internal structures
  We use a data set of nearby galaxies drawn from the HERACLES, ATLAS3D, and
COLD GASS surveys to study variations in molecular gas depletion time (Tdep) in
galaxy structures such as bulges, grand-design spiral arms, bars and rings.
Molecular gas is traced by CO line emission and star formation rate (SFR) is
derived using the combination of far-ultraviolet and mid-infrared (MIR) data.
The contribution of old stars to MIR emission for the ATLAS3D sample is
corrected using 2MASS K-band images. We apply a two-dimensional image
decomposition algorithm to decompose galaxies into bulges and discs. Spiral
arms, bars and rings are identified in the residual maps, and molecular gas
depletion times are derived on a square grid of 1 kpc^2 size. In previous work,
we showed that Tdep correlates strongly with specific star formation rate
(sSFR). We now find that at a given sSFR, the bulge has shorter Tdep than the
disc. The shift to shorter depletion times is most pronounced in the inner
bulge (R < 0.1Re). Grids from galaxies with bars and rings are similar to those
from galactic bulges in that they have reduced Tdep at a given sSFR. In
contrast, the Tdep versus sSFR relation in the discs of galaxies with spiral
arms is displaced to longer Tdep at fixed sSFR. We then show that the
differences in the Tdep-sSFR relation for bulges, discs, arms, bars and rings
can be linked to variations in "stellar", rather than gas surface density
between different structures. Our best current predictor for Tdep, both
globally and for 1 kpc grids, is given by Tdep=
-0.36log(Sigma_SFR)-0.5log(Sigma_*)+5.87.
