MUSEQuBES: The column density, covering fraction, and mass of OVI-bearing gas in and around low-redshift galaxies

Abstract

We present a study of OVI-bearing gas around 247 low-mass (median log(M*/Msun)~8.7) galaxies at low redshifts (0.1 < z < 0.7) using background quasars as part of the MUSE Quasar-fields Blind Emitters Survey (MUSEQuBES). We find that the average OVI column density, ${\rm log}_{10}<N({\rm OVI})/{\rm cm}^{-2}>$ = $14.14^{+0.09}_{-0.10}$, measured within the virial radius for our sample, is significantly lower than for L_* galaxies. Combining 253 star-forming galaxies (mostly more massive) from the literature with 176 star-forming galaxies from MUSEQuBES, we find that both <N(OVI)> and the average covering fraction peak at log(M*/Msun)~9.5. The virial temperature corresponding to this stellar mass is ideal for OVI production via collisional ionization. However, we argue that photoionization and/or non-equilibrium processes are necessary to produce the OVI associated with low-mass, dwarf galaxies (log(M*/Msun)<9). The average OVI mass within the virial radius of dwarf galaxies is measured to be $10^{5.2_{-0.1}^{+0.1}}$ Msun. The characteristic normalized impact parameter at which the OVI covering fraction drops to half of its peak value is the largest (~1.1) for galaxies with stellar mass log(M*/Msun)~9.5. We report the presence of a highly ionized metal floor with ${\rm log}_{10}(N({\rm OVI})/{\rm cm}^{-2}) = 13.2$ outside the virial radius of dwarf galaxies inferred from median spectral stacking.