Bimodality of low-redshift circumgalactic O vi in non-equilibrium eagle zoom simulations

We introduce a series of 20 cosmological hydrodynamical simulations of L* (M 200 = 1011.7–1012.3 M⊙) and group-sized (M 200 = 1012.7–1013.3 M⊙) haloes run with the model used for the eagle project, which additionally includes a non-equilibrium ionization and cooling module that follows 136 ions. The simulations reproduce the observed correlation, revealed by COS-Halos at z ∼ 0.2, between ${{\rm O} \small{VI}}$ column density at impact parameters b < 150 kpc and the specific star formation rate (sSFR ≡ SFR/M *) of the central galaxy at z ∼ 0.2. We find that the column density of circumgalactic ${O \small {VI}}$ is maximal in the haloes associated with L* galaxies, because their virial temperatures are close to the temperature at which the ionization fraction of ${O \small {VI}}$ peaks (T ∼ 105.5 K). The higher virial temperature of group haloes (>106 K) promotes oxygen to higher ionization states, suppressing the ${O \small {VI}}$ column density. The observed $N_{\rm O\,\small {VI}}$ –sSFR correlation therefore does not imply a causal link, but reflects the changing characteristic ionization state of oxygen as halo mass is increased. In spite of the mass dependence of the oxygen ionization state, the most abundant circumgalactic oxygen ion in both L* and group haloes is ${O\,{\small VII}}$ ; ${O \small {VI}}$ accounts for only 0.1 per cent of the oxygen in group haloes and 0.9–1.3 per cent with L* haloes. Nonetheless, the metals traced by ${O \small {VI}}$ absorbers represent a fossil record of the feedback history of galaxies over a Hubble time; their characteristic epoch of ejection corresponds to z > 1 and much of the ejected metal mass resides beyond the virial radius of galaxies. For both L* and group galaxies, more of the oxygen produced and released by stars in the circumgalactic medium (within twice the virial radius) than in the stars and interstellar medium of the galaxy.