The ionization and metallicity of the intervening OVI absorber at z=0.1212 in the spectrum of H1821+643

We use high-resolution UV spectra of the radio-quiet quasar (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 O VI 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 H I Ly alpha absorption lines near the O VI redshift, including at least five components spread over a velocity range of similar to 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 approximate to 85 km s(-1), (4) the O VI absorption is not aligned with the strongest (saturated) H I absorption, but instead is well aligned with the very broad component, and (5) the only detected species (at the 4 sigma level) are O VI and H I, despite coverage of strong transitions of abundant elements (e.g., C II, CIII, and C IV). Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with 10(5.3) less than or equal to T less than or equal to 10(5.6) K and -1.8 less than or equal to [O/ H] less than or equal to -0.6. However, we find that photoionization is also viable if the path length l through the absorbing gas is long enough; simple photoionization models require 85 less than or equal to l less than or equal to 1900 kpc and -1.1 less than or equal to [O/H] less than or equal to -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy, and we conclude with some comments regarding the nature of O VI absorbers.