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 HERA CO-Line Extragalactic Survey, ATLAS^3D, and COLD GASS surveys to study variations in molecular gas depletion time (t_dep) 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 ATLAS^3D 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 kpc² size. In previous work, we showed that t_dep correlates strongly with specific star formation rate (sSFR). We now find that at a given sSFR, the bulge has shorter t_dep than the disc. The shift to shorter depletion times is most pronounced in the inner bulge (R < 0.1 R_e). Grids from galaxies with bars and rings are similar to those from galactic bulges in that they have reduced t_dep at a given sSFR. In contrast, the t_dep versus sSFR relation in the discs of galaxies with spiral arms is displaced to longer t_dep at fixed sSFR. We then show that the differences in the t_dep-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 t_dep, both globally and for 1 kpc grids, is given by t_dep = -0.36 log (Σ_SFR) - 0.50 log (Σ_*) + 5.87.