The Ionization and Metallicity of the Intervening O VI absorber AT z = 0.1212 in the Spectrum of H1821+643

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We use high-resolution UV spectra of the radio-quiet quasar (QSO) H1821+643 (zem = 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 zabs = 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-175 kpc, (2) there is a complex cluster of H I Lyα absorption lines near the O VI redshift, including at least five components spread over a velocity range of ∼700 km s-1, (3) the strongest Lyα 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 ≈ 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 σ level) are O VI and H I, despite coverage of strong transitions of abundant elements (e.g., C II, C II, and C IV). Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with 105,3 ≤ T ≤ 105,6 K and -1.8 ≤ [O/ H] ≤ -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 ≤ l ≤ 1900 kpc and -1.1 ≤ [O/H] ≤ -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. Subject headings: intergalactic medium - quasars: absorption lines -quasars: individual (H1821+643).