What determines the HI gas content in galaxies?: morphological
  dependence of the HI gas fraction across M*-SFR plane
  We perform a stacking analysis of the HI spectra from the Arecibo Legacy Fast
ALFA (ALFALFA) survey for optically-selected local galaxies from the Sloan
Digital Sky Survey (SDSS) 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 higher gas fraction, which is consistent with many
previous studies. Then we investigate the morphological dependence of the HI
gas mass fraction at fixed $M_*$ and SFR to minimize the effects of these
parameters. We use three morphological classifications based on parametric
indicator (S\'{e}rsic index), non-parametric 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 HI gas mass
fraction at fixed $M_*$ and SFR when we use C-index. In comparison, there
exists a hint of diminishment in the HI gas mass fraction for "smooth" galaxies
compared with "non-smooth" 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 HI gas content.
