Does Circumgalactic OVI Trace Low-pressure Gas Beyond the Accretion Shock? Clues from HI and Low-ion Absorption, Line Kinematics, and Dust Extinction

Large O VI columns are observed around star-forming low-redshift similar to L* galaxies, with a dependence on impact parameter indicating that most O5+ particles reside beyond half the halo virial radius (greater than or similar to 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 EB-V approximate to 10(-3), and a linear relation between the OVI column and the O VI velocity width. We contrast these observations with two physical scenarios: (1) OVI traces high-pressure (similar to 30 cm(-3) K) collisionally ionized gas cooling from a virially shocked phase, and (2) OVI traces low-pressure (less than or similar to 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 greater than or similar to 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 greater than or similar to Z(circle dot)/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 similar to 5 M-circle dot yr(-1), comparable to the star formation rate of the central galaxies. The OVI line widths are consistent with the velocity shear expected within this flow. The low-pressure scenario predicts a bimodality in absorption line ratios at similar to 100 kpc, due to the pressure jump across the accretion shock.