A STIS survey for o VI absorption systems at 0.12 < z ≲ 0.5.: I.: The statistical properties of ionized gas

We have conducted a systematic survey for intervening O VI absorbers in available echelle spectra of 16 QSOs at z(QSO) 0: 17Y0: 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)greater than or similar to 25 m angstrom. Ten of these QSOs exhibit strong O VI absorbers in their vicinity. Our O VI survey does not require the known presence of Ly alpha, 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, Delta 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, and 80 m8, respectively, leading to a number density of dN(W >= 50 m angstrom)/ dz 6: 7 1: 7 and dN(W 30 m angstrom)/ dz 10: 4 2: 2. In contrast, we also measure dN/ dz 27 9 for O vi absorbers of W-r > 50 m angstrom at j vj < 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 ( 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.