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 } \ll z _ { QSO } ) O vi systems comprised of 77 individual components , and we find 14 “ proximate ” systems ( z _ { abs } \approx 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 _ { abs } approaches z _ { QSO } ; we find that dN / dz is \approx 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 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 \approx 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 .