The Cosmological Density and Ionization of Hot Gas: O VI Absorption in Quasar Spectra

We have conducted the first survey for Oxygen VI $lambdalambda 1032,1038$ absorption lines in QSO spectra. We used medium resolution ($R approx 1300$) high signal-to-noise ($approx 20$) Faint Object Spectrograph spectra of 11 QSOs ($0.53leq$zem$leq2.08$) from the Hubble Space Telescope Archive. We use simulated spectra to determine the significance of the line identifications, which lie exclusively in the Lya forest. We found 12 O~VI doublets of which 9 are expected to be real and 6 constitute a uniform sample with both lines exceeding a rest equivalent width of $W_r =0.21$ AA. The number of O~VI doublets per unit redshift at a mean absorption redshift of $z_{ave} = 0.9$ is $langle N(z) angle = 1.0 pm0.6$, which is similar to the density of C~IV and Mg~II absorbers.Comment: 28 pages, 5 figures, style file: aaspp4.sty included. Latex file, style file, and five figures are tarred, compressed and uuencoded using uufile