Does Circumgalactic O vi Trace Low-pressure Gas beyond the Accretion Shock? Clues from H i and Low-ion Absorption, Line Kinematics, and Dust Extinction

Large O vi columns are observed around star-forming low-redshift galaxies, with a dependence on impact parameter indicating that most particles reside beyond half the halo virial radius (). In order to constrain the nature of the gas traced by , we analyze additional observables of the outer halo, namely to O vi column ratios of 1-10, an absence of low-ion absorption, a mean differential extinction of , 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 () collisionally ionized gas cooling from a virially shocked phase, and (2) O vi traces low-pressure () 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 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 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 , 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 , due to the pressure jump across the accretion shock.