We use high-resolution UV spectra of the radio-quiet QSO H1821+643 ( z _ { em } = 0.297 ) , obtained with the Space Telescope Imaging Spectrograph ( STIS ) and the Far Ultraviolet Spectroscopic Explorer ( FUSE ) , to study the ionization and metallicity of an intervening \ion O6 absorption line system at z _ { abs } = 0.1212 .
This absorber has the following notable properties : ( 1 ) Several galaxies are close to the sight line at the absorber redshift , including an actively star-forming galaxy at a projected distance of 144 h _ { 75 } ^ { -1 } kpc .
( 2 ) There is a complex cluster of \ion H1 Ly \alpha absorption lines near the \ion O6 redshift , including at least five components spread over a velocity range of \sim 700 km s ^ { -1 } .
( 3 ) The strongest Ly \alpha line in the cluster appears to be composed of a mildly saturated component with a typical b - value blended with a remarkably broad component with b \approx 85 km s ^ { -1 } .
( 4 ) The \ion O6 absorption is not aligned with the strongest ( saturated ) \ion H1 absorption , but instead is well-aligned with the very broad component .
( 5 ) The only detected species ( at the 4 \sigma level ) are \ion O6 and \ion H1 despite coverage of strong transitions of abundant elements ( e.g. , \ion C2 , \ion C3 , and \ion C4 ) .
Based on these constraints , we find that the absorption line properties can be produced in collisionally ionized gas with 10 ^ { 5.3 } \leq T \leq 10 ^ { 5.6 } K and -1.8 \leq [ O/H ] \leq - 0.6 .
However , we find that photoionization is also viable if the pathlength l through the absorbing gas is long enough ; simple photoionization models require 85 \leq l \leq 1900 kpc and -1.1 \leq [ O/H ] \leq - 0.3 .
We briefly discuss how observations of X-ray absorption lines due to \ion O7 and \ion O8 could be used , in principle , to break the ionization mechanism degeneracy , and we conclude with some comments regarding the nature of \ion O6 absorbers .