A High-resolution Survey of Low-redshift Qso Absorption Lines: Statistics and Physical Conditions of O Vi Absorbers

Using high-resolution ultraviolet spectra of 16 low−z QSOs obtained with the E140M echelle mode of the Space Telescope Imaging Spectrograph, we study the physical conditions and statistics of O VI absorption in the intergalactic medium (IGM) at z < 0.5. We identify 51 intervening (zabs ≪ zQSO) O VI systems comprised of 77 individual components, and we find 14 “proximate” systems (zabs ≈ zQSO) containing 34 components. For intervening systems (components) with rest-frame equivalent width Wr > 30 mÅ, the number of O VI absorbers per unit redshift dN/dz = 15.6 +2.9 −2.4 (21.0 +3.2 −2.8), and this decreases to dN/dz = 0.9 −0.5 (0.3 +0.7 −0.3) for Wr > 300 mÅ. The number per redshift increases steeply as zabs approaches zQSO; we find that dN/dz is ≈ 3− 10 times higher within 2500 km s −1 of zQSO. The most striking difference between intervening and proximate systems is that some proximate absorbers have substantially lower H I/O VI ratios. The lower ratios in proximate systems could be partially due to ionization effects, but these proximate absorbers must also have significantly higher metallicities. We find that 37% of the intervening O VI absorbers have velocity centroids that are well-aligned with corresponding H I absorption. If the O VI and the H I trace the same gas, the relatively small differences in line widths imply the absorbers are cool with T < 10 K. Most of these well-aligned absorbers have the characteristics of metal-enriched photoionized gas. However, the O VI in the apparently simple and cold systems could be associated with a hot phase with T ≈ 10 K if the metallicity is high enough to cause the associated broad Lyα absorption to be too weak to detect. We show that 53% of the intervening O VI systems are complex multiphase absorbers that can accommodate both lower metallicity collisionally-ionized gas with T > 10 K and cold photoionzed gas. Subject headings: cosmology:observations — intergalactic medium — quasars: absorption lines