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Abstract

Large O VI columns are observed around star-forming low-redshift ̃ {L}^* galaxies, with a dependence on impact parameter indicating that most {{{O}}}⁵⁺ particles reside beyond half the halo virial radius (≳ 100 {kpc}). In order to constrain the nature of the gas traced by {{O}} {{vi}}, we analyze additional observables of the outer halo, namely {{H}} {{i}} 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 (̃ 30 {cm}}^-3 {{K}}) collisionally ionized gas cooling from a virially shocked phase, and (2) O VI traces low-pressure (≲ 1 {cm}}^-3 {{K}}) 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 ≳ 100 {kpc} 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 ̃ 5 {{{M}}}_☉ {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 ̃ 100 {kpc}, due to the pressure jump across the accretion shock.