A STIS Survey for O vi Absorption Systems at 0.12

We have conducted a systematic survey for intervening O VI absorbers in available echelle spectra of 16 QSOs at z QSO = 0.17–0.57. These spectra were obtained using HST STIS with the E140M grating. Our search uncovered a total of 27 foreground O VI absorbers with rest-frame absorption equivalent width W r (1031) ≳ 25 mÅ. Ten of these QSOs exhibit strong O VI absorbers in their vicinity. Our O VI survey does not require the known presence of Ly α , and the echelle resolution allows us to identify the O VI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, Δ z , using a series of Monte Carlo simulations, and find that Δ z = 1.66,2.18, and 2.42 for absorbers of strength W r = 30,50, and 80 mÅ, respectively, leading to a number density of d ( W ⩾ 50 mÅ)/ dz = 6.7 ± 1.7 and d ( W ⩾ 30 mÅ)/ dz = 10.4 ± 2.2. In contrast, we also measure d / dz = 27 ± 9 for O VI absorbers of W r > 50 mÅ at | Δ v| < 5000 km s −1 from the background QSOs. Using the random sample of O VI absorbers with well-characterized survey completeness, we estimate a mean cosmological mass density of the O VI gas Ω (O 5 + ) h = (1.7 ± 0.3) × 10 −7 . In addition, we show that <5% of O VI absorbers originate in underdense regions that do not show a significant trace of H I. Furthermore, we show that the neutral gas column N (H I) associated with these O VI absorbers spans nearly 5 orders of magnitude, and shows moderate correlation with N (O VI). Finally, while the number density of O VI absorbers varies substantially from one sight line to another, it also appears to be inversely correlated with the number density of H I absorbers along individual lines of sight.