The role of HI in regulating size growth of local galaxies

TL;DR

This study investigates how atomic hydrogen (HI) influences the size growth of local galaxies, revealing that HI content correlates with galaxy size in HI-rich regimes and suggesting inside-out disk growth driven by gas accretion.

Contribution

It provides new insights into the relationship between HI content and galaxy size growth, emphasizing the role of gas accretion and angular momentum in HI-rich galaxies.

Findings

Galaxy size correlates with HI-to-stellar mass ratio in HI-rich galaxies.

Size growth is minimal in HI-poor galaxies regardless of HI content.

Size is more linked to HI content than star formation rate.

Abstract

We study the role of atomic hydrogen (HI) in regulating size growth of local galaxies. The size of a galaxy, $D_{\rm r,~25}$, is characterized by the diameter at which the $r-$band surface brightness reaches $\mu_{\rm r}=25.0~\rm mag~arcsec^{-2}$. We find that the positions of galaxies in the size ($D_{\rm r,~25}$)$-$stellar mass ($M_{\ast}$) plane strongly depend on their HI-to-stellar mass ratio ($M_{\rm HI}/M_{\ast}$). In the HI-rich regime, galaxies that are more rich in HI tend to have larger sizes. Such a trend is not seen in the HI-poor regime, suggesting that size growth is barely affected by the HI content when it has declined to a sufficiently low level. An investigation of the relations between size, $M_{\rm HI}/M_{\ast}$ and star formation rate (SFR) suggests that size is more intrinsically linked with $M_{\rm HI}/M_{\ast}$, rather than SFR. We further examine the HI-to-stellar disk size ratio ($D_{\rm HI}/D_{\rm r,~25}$) of galaxies and find that at log($M_{\rm HI}/M_{\ast})>-0.7$, $D_{\rm HI}/D_{\rm r,~25}$ is weakly correlated with $M_{\ast}$. These findings support a picture in which the HI-rich galaxies live in an inside-out disk growing phase regulated by gas accretion and star formation. The angular momentum of the accreted materials is probably the key parameter in shaping the size of an HI-rich galaxy.