The Relationship between Galaxy ISM and Circumgalactic Gas Metallicities

Abstract

Abstract We present interstellar matter (ISM) and circumgalactic medium (CGM) metallicities for 25 absorption systems associated with isolated star-forming galaxies ( ) with 9.4 ≤ log(M */M ⊙) ≤ 10.9 and with absorption detected within (200 kpc). Galaxy ISM metallicities were measured using Hα/[N ii] emission lines from Keck/ESI spectra. CGM single-phase low-ionization metallicities were modeled using Markov Chain Monte Carlo and Cloudy analysis of absorption from HST/COS and Keck/HIRES or VLT/UVES quasar spectra. We find that the star-forming galaxy ISM metallicities follow the observed stellar mass–metallicity relation (1σ scatter 0.19 dex). CGM metallicity shows no dependence with stellar mass and exhibits a scatter of ∼2 dex. All CGM metallicities are lower than the galaxy ISM metallicities and are offset by log(dZ) = −1.17 ± 0.11. There is no obvious metallicity gradient as a function of impact parameter or virial radius (<2.3σ significance). There is no relationship between the relative CGM-galaxy metallicity and azimuthal angle. We find the mean metallicity differences along the major and minor axes are −1.13 ± 0.18 and −1.23 ± 0.11, respectively. Regardless of whether we examine our sample by low/high inclination or low/high impact parameter, or low/high N(H i), we do not find any significant relationship with relative CGM-galaxy metallicity and azimuthal angle. We find that 10/15 low column density systems (logN(H i) < 17.2) reside along the galaxy major axis while high column density systems (logN(H i) ≥ 17.2) reside along the minor axis. This suggests N(H i) could be a useful indicator of accretion/outflows. We conclude that CGM is not well mixed, given the range of galaxy-CGM metallicities, and that metallicity at low redshift might not be a good tracer of CGM processes. On the other hand, we should replace integrated line-of-sight, single-phase metallicities with multiphase, cloud–cloud metallicities, which could be more indicative of the physical processes within the CGM.