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Large OVI 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 toconstrain the nature of the gas traced by OVI, we analyze additionalobservables of the outer halo, namely HI to OVI column ratios of 1-10, anabsence of low-ion absorption, a mean differential extinction of E(B-V)~10^-3,and a linear relation between OVI column and velocity width. We contrast theseobservations with two physical scenarios: (1) OVI traces high-pressure (~30cm^-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, wherethe gas is in ionization and thermal equilibrium with the UV background. Wedemonstrate that the high-pressure scenario requires multiple gas phases toexplain the observations, and a large deposition of energy at >~100 kpc tooffset the energy radiated by the cooling gas. In contrast, the low-pressurescenario can explain all considered observations with a single gas phase inthermal equilibrium, provided that the baryon overdensity is comparable to thedark-matter overdensity, and that the gas is enriched to >~Z_sun/3 with anISM-like dust-to-metal ratio. The low-pressure scenario implies that OVI tracesa cool flow with mass flow rate of ~5 M_sun yr^-1, comparable to the starformation rate of the central galaxies. The OVI line widths are consistent withthe velocity shear expected within this flow. The low-pressure scenariopredicts a bimodality in absorption line ratios at ~100 kpc, due to thepressure jump across the accretion shock.

astrophysical journal/the astrophysical journal

Does circumgalactic OVI trace low-pressure gas beyond the accretion  shock? Clues from HI and low-ion absorption, line kinematics, and dust  extinction