The Surprisingly Constant Strength of O VI Absorbers over Cosmic Time
  O VI absorption is observed in a wide range of astrophysical environments,
including the Local ISM, the disk and halo of the Milky Way, high-velocity
clouds, the Magellanic clouds, starburst galaxies, the intergalactic medium,
damped Lyman-alpha 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 datasets 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 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. (2002), 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 (~10^4 K) clouds and hot
(~10^6 K) plasma, although many such regions would have to be intersected by a
typical galaxy-halo sightline 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.
