CGM² I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen

We present initial results from the COS and Gemini Mapping the Circumgalactic Medium (CGMCGM = CGM²) survey. The CGM² spectroscopic survey consists of 1689 galaxies, all with high-quality Gemini GMOS spectra suitable for precise redshift measurements, within 1 Mpc of twenty-two z < 1 quasars, all with existing S/N ~ 10 HST-COS G130M+G160M spectra. We show that the HI covering fraction above a threshold of log(NHI) = 14 is greater than 60% within 1.5 virial radii (Rvir ~ R200m) of galaxies having stellar masses 10^7-10 Msun. We examine the kinematics of the HI absorption relative to the galaxy systemic velocities and find that the majority of absorption lies within ± 250 km/s, and that this concentration increases with increasing HI column density from log(NHI) = 14-17. We examine HI covering fractions over a range of impact parameters to infer a characteristic size of the CGM, R¹⁴_CGM, as a function of galaxy mass. Our analysis suggests that using Rvir as a proxy for the characteristic radius of the CGM likely underestimates its extent, especially for galaxies with M* < 10¹⁰ Msun.