2011ApJ...730...58F

O VI absorption is observed in a wide range of astrophysical environments, including the local interstellar medium, the disk and halo of the Milky Way, high-velocity clouds, the Magellanic Clouds, starburst galaxies, the intergalactic medium (IGM), damped Lyα systems, and gamma-ray-burst host galaxies. Here, a new compilation of 775 O VI absorbers drawn from the literature is presented, all observed at high resolution (instrumental FWHM ≤ 20 km/s) and covering the redshift range z = 0-3. In galactic environments [log N(H I) ≳ 20], the mean O VI column density is shown to be insensitive to metallicity, taking a value log N(O VI) ~ 14.5 for galaxies covering the range -1.6 ≲ [O/H] ≲ 0. In intergalactic environments [log N(H I) < 17], the mean O VI component column density measured in data sets of similar sensitivity shows only weak evolution between z = 0.2 and z = 2.3, but IGM O VI components are on average almost twice as broad at z = 0.2 than at z = 2.3. The implications of these results on the origin of O VI are discussed. The existence of a characteristic value of log N(O VI) for galactic O VI absorbers, and the lack of evolution in log N(O VI) for intergalactic absorbers, lend support to the "cooling-flow" model of Heckman et al., in which all O VI absorbers are created in regions of initially hot shock-heated plasma that are radiatively cooling through coronal temperatures. These regions could take several forms, including conductive, turbulent, or shocked boundary layers between warm (∼10⁴ K) clouds and hot (∼10⁶ K) plasma, although many such layers would have to be intersected by a typical galaxy-halo sight line to build up the characteristic galactic N(O VI). The alternative, widely used model of single-phase photoionization for intergalactic O VI is ruled out by kinematic evidence in the majority of IGM O VI components at low and high redshift.