The variation in molecular gas depletion time among nearby galaxies: II the impact of galaxy internal structures

TL;DR

This study investigates how molecular gas depletion time varies across different galaxy structures, revealing that bulges, bars, and rings have shorter depletion times than discs at the same star formation rate, influenced mainly by stellar surface density.

Contribution

It provides a detailed analysis of how galaxy internal structures affect molecular gas depletion times, highlighting the role of stellar surface density over gas density.

Findings

Bulges, bars, and rings have shorter Tdep than discs at fixed sSFR.

Inner bulges show the most pronounced reduction in Tdep.

Stellar surface density is a better predictor of Tdep than gas surface density.

Abstract

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.