What determines the HI gas content in galaxies?: morphological dependence of the HI gas fraction across M*-SFR plane

TL;DR

This study uses stacking analysis of HI spectra from the ALFALFA survey to explore how galaxy morphology influences the HI gas fraction at fixed stellar mass and SFR, revealing that small-scale structures may be linked to gas content.

Contribution

It provides new insights into the morphological dependence of HI gas content, especially highlighting the role of small-scale structures visible in optical images.

Findings

No significant morphological dependence when using C-index.

Hint of lower HI gas fraction in 'smooth' galaxies compared to 'non-smooth'.

Visual smoothness correlates with small-scale structures affecting HI content.

Abstract

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.