The cosmological density and ionization of hot gas: oxygen VI absorption in quasar spectra

We have conducted the first survey for Oxygen VI \lambda\lambda 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 (0.53\leq\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)\rangle = 1.0 \pm0.6, which is similar to the density of C~IV and Mg~II absorbers