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 (z_abs ≪ Z_QSO) O VI systems comprising 77 individual components, and we find 14 "proximate" systems (z_abs ≈ Z_QSO) containing 34 components. For intervening systems (components) with rest-frame equivalent width W_r > 30 mÅ, the number of O VI absorbers per unit redshift dN/dz = 15.6_-2.4^+2.9 (21.0 _-2.8^+3.2), and this decreases to dN/dz = 0.9 _-0.5^+1.0 (0.3_-0.3^+0.7) for W _r > 300 mÅ. The number per redshift increases steeply as z_abs approaches Z_QSO; we find that dN/dz is ≈3-10 times higher within 2500 km s^-1 of Z_QSO. 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 that the absorbers are cool, with T < 10⁵ 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 ≈ 10^5.5 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 > 10⁵ K and cold photoionzed gas.