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 $M_{\ast }\sim {10}^{10}$ $M_{\odot }$ galaxies. The galaxies have redshifts $0.12<z<0.6$, stellar masses ${10}^{10.1}<M_{\ast }<{10}^{10.9}$ $M_{\odot }$, and spectroscopic classifications as star-forming or passive. We compare the incidence rates of high column density O VI absorption ($N_{\mathrm{O}\mathrm{V}\mathrm{I}}\ge {10}^{14.3}$ 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 $M_{\ast }\sim {10}^{10}$ $M_{\odot }$ 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.