2008ApJS..177...39T

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^+2.9_–2.4(21.0^+3.2_–2.8), and this decreases to dN/dz=0.9^+1.0_–0.5(0.3^+0.7_–0.3) for W_r>300 mÅ. The number per redshift increases steeply as z_absapproaches z_QSO; we find that dN/dz is ~3-10 times higher within 2500 km/s 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.