Cool CGM MgII Absorption Across the Star-Forming, Green Valley, and Quiescent Transition

Abstract

We investigate the distribution and kinematics of cool circumgalactic medium (CGM) gas across the star-forming to quiescent transition using MgII absorption for 716 galaxies spanning $0.07<z<2.7$ (169 new and 547 archival galaxies). To compare galaxies uniformly across 10 billion years of cosmic time, we introduce a star-formation offset metric (${\sigma }_{\mathrm{SFO}}$), which measures a galaxy's deviation from the star-forming main sequence of its epoch. A key advantage of ${\sigma }_{\mathrm{SFO}}$ is its ability to identify transitional green valley galaxies as a distinct population across redshift, which would otherwise be obscured by binary star-forming-passive classifications. We fit a virial-radius-normalized radial profile and find that the MgII absorption strength declines with increasing projected distance from the host galaxy. The scatter around this mean profile correlates strongly with star-formation activity: radial profile residuals correlate positively with $\log sSFR$ and ${\sigma }_{\mathrm{SFO}}$, with star-forming galaxies showing excess absorption above the profile, green-valley galaxies showing intermediate residuals, and quiescent galaxies falling preferentially below it. The MgII covering fraction in the inner CGM ($R/R_{200}<0.25$) follows the same sequence, declining monotonically from star-forming through green-valley to quiescent systems. MgII absorption kinematics are consistent with a predominantly bound cool CGM. Star-forming galaxies further exhibit a bimodal azimuthal dependence, with MgII absorption enhanced along both the polar and disk directions, consistent with bipolar outflows and co-planar accretion. Together, these results indicate that the cool CGM tracks the quenching of star-formation in galaxies, with ${\sigma }_{\mathrm{SFO}}$ revealing a gradual decline in cool gas across the green valley rather than an abrupt star-forming-to-passive transition.

25 pages, 14 figures, submitted to ApJ