The role of HI in regulating size growth of local galaxies
  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.
