What Determines the H i Gas Content in Galaxies? Morphological Dependence of the H i Gas Fraction across the M –SFR Plane

We perform a stacking analysis of the H i spectra from the Arecibo Legacy Fast ALFA survey for optically selected local galaxies from the Sloan Digital Sky Survey to study the average gas fraction of galaxies at fixed stellar mass ( M * ) and star formation rate (SFR). We first confirm that the average gas fraction strongly depends on the stellar mass and SFR of host galaxies; massive galaxies tend to have a lower gas fraction, and actively star-forming galaxies show a higher gas fraction, which is consistent with many previous studies. Then we investigate the morphological dependence of the H i gas mass fraction at fixed M * and SFR to minimize the effects of these parameters. We use three morphological classifications based on parametric indicator (Sérsic index), nonparametric indicator (C-index), and visual inspection (smoothness from the Galaxy Zoo 2 project) on the optical image. We find that there is no significant morphological dependence of the H i gas mass fraction at fixed M * and SFR when we use the C-index. In comparison, there exists a hint of diminishment in the H i gas mass fraction for “smooth” galaxies compared with “nonsmooth” galaxies. We find that the visual smoothness is sensitive to the existence of small-scale structures in a galaxy. Our result suggests that even at fixed M * and SFR, the presence of such small-scale structures (seen in the optical image) is linked to their total H i gas content.