Does circumgalactic OVI trace low-pressure gas beyond the accretion
  shock? Clues from HI and low-ion absorption, line kinematics, and dust
  extinction
  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 to
constrain the nature of the gas traced by OVI, we analyze additional
observables of the outer halo, namely HI to OVI 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 OVI column and 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_sun/3 with an
ISM-like dust-to-metal ratio. The low-pressure scenario implies that OVI traces
a cool flow with mass flow rate of ~5 M_sun 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.
