A DEEP SEARCH for FAINT GALAXIES ASSOCIATED with VERY LOW REDSHIFT C IV ABSORBERS. III. the MASS- and ENVIRONMENT-DEPENDENT CIRCUMGALACTIC MEDIUM

Using Hubble Space Telescope Cosmic Origins Spectrograph observations of 89 QSO sightlines through the Sloan Digital Sky Survey footprint, we study the relationships between C iv absorption systems and the properties of nearby galaxies, as well as the large-scale environment. To maintain sensitivity to very faint galaxies, we restrict our sample to 0.0015 < z < 0.015, which defines a complete galaxy survey to L ≳ 0.01 L_z.ast; or stellar mass M_∗ ≳ 10⁸ M_⊙. We report two principal findings. First, for galaxies with impact parameter ρ < 1 r_vir, C iv detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C iv is preferentially associated with galaxies with M_∗ > 10^9.5 M_⊙; lower-mass galaxies rarely exhibit significant C iv absorption (covering fraction fc = 9⁺¹² _-6% for 11 galaxies with M_∗ < 10^9.5 M_⊙). Second, C iv detection within the M_∗ > 10^9.5 M_⊙ population depends on environment. Using a fixed-aperture environmental density metric for galaxies with ρ < 160 kpc at z < 0.055, we find that 57⁺¹² _-13% (8/14) of galaxies in low-density regions (regions with fewer than seven L > 0.15 L_∗ galaxies within 1.5 Mpc) have affiliated C iv absorption; however, none (0/7) of the galaxies in denser regions show C iv. Similarly, the C iv detection rate is lower for galaxies residing in groups with dark matter halo masses of M_halo > 10^12.5 M_⊙. In contrast to C iv, H i is pervasive in the circumgalactic medium without regard to mass or environment. These results indicate that C iv absorbers with log N (C IV) ≳ 13.5 cm^-2 trace the halos of M_∗ > 10^9.5 M_⊙ galaxies but also reflect larger-scale environmental conditions.