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 (& 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 ≈ 10−3, and a linear relation between O vi column and 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 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.
|Original language||English (US)|
|State||Published - Mar 14 2018|
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