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.