Signatures of gas flows-I: Connecting the kinematics of the HI circumgalactic medium to galaxy rotation

Abstract

The CGM hosts many physical processes with different kinematic signatures that affect galaxy evolution. We address the CGM-galaxy kinematic connection by quantifying the fraction of HI that is aligned with galaxy rotation with the equivalent width co-rotation fraction, $f_{\mathrm{EWcorot}}$. Using 70 quasar sightlines having HST/COS HI absorption ($12<\log (N(HI)/{\mathrm{cm}}^{-2})<20$) within $5R_{\mathrm{vir}}$ of $z<0.6$ galaxies we find that $f_{\mathrm{EWcorot}}$ increases with increasing HI column density. $f_{\mathrm{EWcorot}}$ is flat at $\sim 0.6$ within $R_{\mathrm{vir}}$ and decreases beyond $R_{\mathrm{vir}}$ to $f_{\mathrm{EWcorot}}$$\sim 0.35$. $f_{\mathrm{EWcorot}}$ also has a flat distribution with azimuthal and inclination angles within $R_{\mathrm{vir}}$, but decreases by a factor of two outside of $R_{\mathrm{vir}}$ for minor axis gas and by a factor of two for edge-on galaxies. Inside $R_{\mathrm{vir}}$, co-rotation dominated HI is located within $\sim 20$ deg of the major and minor axes. We surprisingly find equal amounts of HI absorption consistent with co-rotation along both major and minor axes within $R_{\mathrm{vir}}$. However, this co-rotation disappears along the minor axis beyond $R_{\mathrm{vir}}$, suggesting that if this gas is from outflows, then it is bound to galaxies. $f_{\mathrm{EWcorot}}$ is constant over two decades of halo mass, with no decrease for log(M${}_{\mathrm{h}}/M_{\odot })>12$ as expected from simulations. Our results suggest that co-rotating gas flows are best found by searching for higher column density gas within $R_{\mathrm{vir}}$ and near the major and minor axes.

22 pages, 11 figures, accepted for publication in MNRAS July 26th 2024