A high-resolution survey of low-redshift QSO absorption lines: Statistics and physical conditions of OVI 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) approximate to z(QSO)) containing 34 components. For intervening systems (components) with rest-frame equivalent width W-r > 30 m angstrom, the number of O VI absorbers per unit redshift dN/dz = 15.6(-2.9)(+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 m angstrom. The number per redshift increases steeply as z(abs) approaches z(QSO); we find that dN/dz is approximate to 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 lowerHI/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 HI absorption. If the O VI and the HI trace the same gas, the relatively small differences in line widths imply that the absorbers are cool, with T < 10(5) 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 approximate to 10(5.5) K if the metallicity is high enough to cause the associated broad Ly alpha 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(5) K and cold photoionzed gas.