Signatures of gas flows – I. Connecting the kinematics of the H i 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 H i that is aligned with galaxy rotation with the equivalent width co-rotation fraction, fEWcorot. Using 70 quasar sightlines having HST/COS H i absorption (12 < log (N(H i)/cm‒2) < 20) within 5Rvir of z < 0.6 galaxies we find that fEWcorot increases with increasing H i column density. fEWcorot is flat at ∼0.6 within Rvir and decreases beyond Rvir to fEWcorot∼0.35. fEWcorot also has a flat distribution with azimuthal and inclination angles within Rvir, but decreases by a factor of two outside of Rvir for minor axis gas and by a factor of two for edge-on galaxies. Inside Rvir, co-rotation dominated H i is located within ∼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 Rvir. However, this co-rotation disappears along the minor axis beyond Rvir, suggesting that if this gas is from outflows, then it is bound to galaxies. fEWcorot is constant over two decades of halo mass, with no decrease for log(Mh/M⊙) > 12 as expected from simulations. Our results suggest that co-rotating gas flows are best found by searching for higher column density gas within Rvir and near the major and minor axes.