# 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

Published in ApJ, 2018

Recommended citation: Stern, J., Faucher-Giguère, C.-A., Hennawi, J., Hafen, Z., Johnson, S., Fielding, Drummond (2018). "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." ApJ, 865(2):91. http://dfielding14.github.io/files/Stern18_lowP.pdf

Large OVI 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 halOVIrial radius (≳ 100 kpc). In order to constrain the nature of the gas traced by OVI, we analyze additional observables of the outer halo, namely H i to OVI 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 the OVI column and the OVI velocity width. We contrast these observations with two physical scenarios: (1) OVI traces high-pressure (~ 30 cm-3 K) collisionally ionized gas cooling from a virially shocked phase, and (2) OVI 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 OVI 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 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 ̃ 100 kpc, due to the pressure jump across the accretion shock.