The role of HI in regulating size growth of local galaxies

Abstract

We study the role of atomic hydrogen (HI) in regulating size growth of local galaxies. The size of a galaxy, $D_{\mathrm{r},25}$, is characterized by the diameter at which the $r-$band surface brightness reaches ${\mu }_{\mathrm{r}}=25.0\mathrm{mag}\mathrm{arcse}{\mathrm{c}}^{-2}$. We find that the positions of galaxies in the size ($D_{\mathrm{r},25}$)$-$stellar mass ($M_{\ast }$) plane strongly depend on their HI-to-stellar mass ratio ($M_{\mathrm{HI}}/M_{\ast }$). 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, $M_{\mathrm{HI}}/M_{\ast }$ and star formation rate (SFR) suggests that size is more intrinsically linked with $M_{\mathrm{HI}}/M_{\ast }$, rather than SFR. We further examine the HI-to-stellar disk size ratio ($D_{\mathrm{HI}}/D_{\mathrm{r},25}$) of galaxies and find that at log($M_{\mathrm{HI}}/M_{\ast })>-0.7$, $D_{\mathrm{HI}}/D_{\mathrm{r},25}$ 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.

13 pages, 12 figures, accepted by ApJ

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