Evidence for galaxy quenching in the green valley caused by a lack of a circumgalactic medium

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°) disc galaxy (G1547) where the CGM is probed by a background quasar at a distance of 84 kpc and within 10° of the galaxy major axis. G1547 does not have any detectable CGM absorption down to stringent limits, covering H I (EW_r <0.02 Å, log(N(H I)/cm^-2) < 12.6) and a range of low and high ionization absorption lines (O I, C II, N II, Si II, C III, N III, Si III, C IV, Si IV, N V, and O VI). 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 star formation rate (SFR, 1.1 M_⊙ yr^-1), specific SFR (sSFR, 1.5 × 10^-10 yr^-1), and Σ_SFR (0.06 M_⊙ yr^-1 kpc^-2) and does not exhibit active galactic nucleus 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 H I absorption-selected galaxies, we find that quiescent galaxies with log(sSFR/yr^-1) < -11 have a low probability (4/12) of possessing detectable H I in their CGM, while all galaxies (40/40) with log(sSFR/yr^-1) > -11 have H I absorption. We conclude that sSFR is a good indicator of the presence of H I 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 time-scale of 4 ± 1 Gyr. G1547 provides a unique perspective into the external mechanisms that could explain the migration of galaxies into the green valley.