A Stis Survey for Ovi Absorption Systems at 0 . 12 < Z . 0 . 5 I . : the Statistical Properties of Ionized Gas 1

We have conducted a systematic survey for intervening OVI absorbers in available echelle spectra of 16 QSOs at zQSO = 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 Wr(1031) & 25 mÅ. Ten of these QSOs exhibit strong OVI absorbers in their vicinity. Our OVI survey does not require the known presence of Lyα, and the echelle resolution allows us to identify the OVI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, ∆ z, using a series of Monte-Carlo simulations, and find that ∆ z = 1.66, 2.18, and 2.42 for absorbers of strength Wr = 30, 50, 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 OVI absorbers of Wr > 50 mÅ at |∆ v| < 5000 km s from the background QSOs. Using the random sample of OVI absorbers with well characterized survey completeness, we estimate a mean cosmological mass density of the OVI gas Ω(O)h = (1.7 ± 0.3) × 10. In addition, we show that < 5% OVI 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 OVI absorbers spans nearly five orders of magnitude, and show moderate correlation with N(OVI). Finally, while the number density of OVI absorbers varies substantially from one sightline to another, it also appears to be inversely correlated with the number density of H I absorbers along individual lines of sight. Subject headings: cosmology: observations—intergalactic medium—quasars: absorption lines