2018ApJ...865...91S

Large O VI columns are observed around star-forming low-redshift ∼L^* galaxies, with a dependence on impact parameter indicating that most O⁵⁺ particles reside beyond half the halo virial radius (≳100kpc). In order to constrain the nature of the gas traced by Ovi, we analyze additional observables of the outer halo, namely Hi to O VI column ratios of 1-10, an absence of low-ion absorption, a mean differential extinction of E_B–V~10^–3, and a linear relation between the O VI column and the O VI velocity width. We contrast these observations with two physical scenarios: (1) O VI traces high-pressure (∼30cm^–3K) collisionally ionized gas cooling from a virially shocked phase, and (2) O VI traces low-pressure (≲1cm^–3K) gas beyond the accretion shock, where the gas is in ionization and thermal equilibrium with the UV background. We demonstrate that the high-pressure scenario requires multiple gas phases to explain the observations and a large deposition of energy at ≳100kpc to offset the energy radiated by the cooling gas. In contrast, the low-pressure scenario can explain all considered observations with a single gas phase in thermal equilibrium, provided that the baryon overdensity is comparable to the dark-matter overdensity and that the gas is enriched to ≳Z_☉/3 with an ISM-like dust-to-metal ratio. The low-pressure scenario implies that O VI traces a cool flow with a mass flow rate of ∼5M_☉yr^–1, comparable to the star formation rate of the central galaxies. The O VI line widths are consistent with the velocity shear expected within this flow. The low-pressure scenario predicts a bimodality in absorption line ratios at ∼100kpc, due to the pressure jump across the accretion shock.