Signatures of gas flows – I. Connecting the kinematics of the H i circumgalactic medium to galaxy rotation

Abstract

ABSTRACT The circumgalactic medium (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 H i that is aligned with galaxy rotation with the equivalent width co-rotation fraction, $f_{\rm EWcorot}$. Using 70 quasar sightlines having Hubble Space Telescope/Cosmic Origins Spectrograph H i absorption (${12\lt \log (N(\rm{{\rm H}\,{\small I}})/{\rm cm}^{-2})\lt 20}$) within $5R_{\rm vir}$ of $z\lt 0.6$ galaxies we find that $f_{\rm EWcorot}$ increases with increasing H i column density. $f_{\rm EWcorot}$ is flat at $\sim 0.6$ within $R_{\rm vir}$ and decreases beyond $R_{\rm vir}$ to $f_{\rm EWcorot}$$\sim 0.35$. $f_{\rm EWcorot}$ also has a flat distribution with azimuthal and inclination angles within $R_{\rm vir}$, but decreases by a factor of two outside of $R_{\rm vir}$ for minor axis gas and by a factor of 2 for edge-on galaxies. Inside $R_{\rm vir}$, co-rotation dominated H i is located within $\sim 20$ deg of the major and minor axes. We surprisingly find equal amounts of H i absorption consistent with co-rotation along both major and minor axes within $R_{\rm vir}$. However, this co-rotation disappears along the minor axis beyond $R_{\rm vir}$, suggesting that if this gas is from outflows, then it is bound to galaxies. $f_{\rm EWcorot}$ is constant over two decades of halo mass, with no decrease for log(M$_{\rm h}/{\rm M}_{\odot })\gt 12$ as expected from simulations. Our results suggest that co-rotating gas flows are best found by searching for higher column density gas within $R_{\rm vir}$ and near the major and minor axes.