Evidence for galaxy quenching in the green valley caused by a lack of a circumgalactic medium
Abstract
The relationship between a galaxy's properties and its circumgalactic medium (CGM) provides a unique view of how galaxies evolve. We present an interesting edge-on (i=86 degrees) disk galaxy (G1547) where the CGM is probed by a background quasar at a distance of 84 kpc and within 10 degrees of the galaxy major axis. G1547 does not have any detectable CGM absorption down to stringent limits, covering HI (EWr<0.02A, log(N(HI)/cm-2)<12.6) and a range of low and high ionisation absorption lines (OI, CII, NII, SiII, CIII, NIII, SiIII, CIV, SiIV, NV and OVI). This system is rare, given the covering fraction of 1.00-0.04+0.00 for sub-L* galaxies within 50-100 kpc of quasar sightlines. G1547 has a low SFR (1.1 Myr-1), SSFR (1.5×10-10yr-1) and SFR (0.06 Myr-1kpc-2) and does not exhibit AGN or star-formation driven outflows. Compared to the general population of galaxies, G1547 is in the green valley and has an above average metallicity with a negative gradient. When compared to other HI absorption-selected galaxies, we find that quiescent galaxies with log(SSFR/yr-1)<-11 have a low probability (4/12) of possessing detectable HI in their CGM, while all galaxies (40/40) with log(SSFR/yr-1)>-11 have HI absorption. We conclude that SSFR is a good indicator of the presence of HI CGM. Interestingly however, G1547 is the only galaxy with log(SSFR/yr-1)>-11 that has no detectable CGM. Given the properties of G1547, and its absent CGM, it is plausible that G1547 is undergoing quenching due to a lack of accreting fuel for star-formation, with an estimated quenching timescale of 41 Gyr. G1547 provides a unique perspective into the external mechanisms that could explain the migration of galaxies into the green valley.