2008ApJ...683...22T

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 Lyα, 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, Δz, using a series of Monte Carlo simulations, and find that Δz=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 dN(W≥50 mÅ)/dz=6.7±1.7 and dN(W≥30 mÅ)/dz=10.4±2.2. In contrast, we also measure dN/dz=27±9 for O VI absorbers of W_r>50 mÅ at|Δv|<5000 km/s 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(O⁵⁺)h=(1.7±0.3)x10^–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.