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 $L\gtrsim 0.01L\ast$ or stellar mass $M_{\ast }\gtrsim {10}^8{M}_{\odot }$. We report two principal findings. First, for galaxies with impact parameter $\rho <1r_{vir}$, C iv detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C iv is preferentially associated with galaxies with $M_{\ast }>{10}^{9.5}{M}_{\odot };$ lower-mass galaxies rarely exhibit significant C iv absorption (covering fraction $f_C=9_{-6}^{+12}\%$ for 11 galaxies with $M_{\ast }<{10}^{9.5}{M}_{\odot }$). Second, C iv detection within the $M_{\ast }>{10}^{9.5}{M}_{\odot }$ population depends on environment. Using a fixed-aperture environmental density metric for galaxies with ρ < 160 kpc at $z<0.055$, we find that ${57}_{-13}^{+12}\%$ (8/14) of galaxies in low-density regions (regions with fewer than seven $L>0.15L\ast$ 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 $M_{halo}>{10}^{12.5}{M}_{\odot }$. 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 $logN\left(CIV\right)\gtrsim 13.5{cm}^{-2}$ trace the halos of $M_{\ast }>{10}^{9.5}{M}_{\odot }$ galaxies but also reflect larger-scale environmental conditions.