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 \leq 20 km s ^ { -1 } ) and covering the redshift range z =0–3 .
In galactic environments [ log N ( H i ) \gtrsim 20 ] , the mean O vi column density is shown to be insensitive to metallicity , taking a value log N ( O vi ) \approx 14.5 for galaxies covering the range –1.6 \lesssim [ O/H ] \lesssim 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 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 .
( 81 ) , 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 ( \sim 10 ^ { 4 } K ) clouds and hot ( \sim 10 ^ { 6 } K ) plasma , although many such layers 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 .