The surprisingly constant strength of O VI absorbers over cosmic time

Abstract

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 ≤ 20km s-1) and covering the redshift range z = 0-3. In galactic environments [logN(H I) ≳ 20], the mean O VI column density is shown to be insensitive to metallicity, taking a value logN(O VI) 14.5 for galaxies covering the range -1.6 ≲ [O/H] ≲ 0. In intergalactic environments [logN(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 logN(O VI) for galactic O VI absorbers, and the lack of evolution in logN(O VI) for intergalactic absorbers, lend support to the "cooling-flow" model of Heckman etal., 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. © 2011. The American Astronomical Society. All rights reserved.