The CGM2 Survey: Quenching and the Transformation of the Circumgalactic Medium

This study addresses how the incidence rate of strong O VI absorbers in a galaxy's circumgalactic medium (CGM) depends on galaxy mass and, independently, on the amount of star formation in the galaxy. We use Hubble Space Telescope/Cosmic Origins Spectrograph absorption spectroscopy of quasars to measure O VI absorption within 400 projected kpc and 300 km s^-1 of 52 galaxies with M _* ~ 3 × 10¹⁰ M _⊙. The galaxies have redshifts 0.12 < z < 0.6, stellar masses 10^10.1 M _⊙ < M _* < 10^10.9 M _⊙, and spectroscopic classifications as star-forming or passive. We compare the incidence rates of high column density O VI absorption (N _{O VI} ≥ 10^14.3 cm^-2) near star-forming and passive galaxies in two narrow ranges of stellar mass and, separately, in a matched range of halo mass. In all three mass ranges, the O VI covering fraction within 150 kpc is higher around star-forming galaxies than around passive galaxies with greater than 3σ-equivalent statistical significance. On average, the CGM of star-forming galaxies with M _* ~ 3 × 10¹⁰ M _⊙ contains more O VI than the CGM of passive galaxies with the same mass. This difference is evidence for a CGM transformation that happens together with galaxy quenching and is not driven primarily by halo mass.