The CGM$^2$ 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 HST/COS absorption spectroscopy of quasars to measure O VI absorption within 400 projected kpc and 300 km s$^{-1}$ of 52 $M_{*}\sim 10^{10}$ $M_\odot$ galaxies. The galaxies have redshifts $0.12<z<0.6$, stellar masses $10^{10.1} < M_* < 10^{10.9}$ $M_\odot$, and spectroscopic classifications as star-forming or passive. We compare the incidence rates of high column density O VI absorption ($N_{\rm O\, VI} \geq 10^{14.3}$ cm$^{-2}$) near star-forming and passive galaxies in two narrow stellar mass ranges and, separately, in a matched halo mass range. 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 $3\sigma$-equivalent statistical significance. On average, the CGM of $M_*\sim 10^{10}$ $M_\odot$ star-forming galaxies 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.