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We have conducted the first survey for QSO with O~VI $\lambda\lambda 1032,1038$ absorption lines. We obtained medium resolution ($R \approx 1300$), high signal-to-noise ($\approx 20$) spectra of 11 QSOs ($0.53\leq z_{em} \leq2.08$) taken with the Faint Object Spectrograph from the Hubble Space Telescope Archive.
The O~VI doublet is found exclusively in the Lyman-$\alpha$ forest.
All previous surveys of metal lines in QSO absorption systems were
done redward of Ly$\alpha$ emission, avoiding blending due to Lyman-$\alpha$
forest clouds. The higher density of lines in the Lyman-$\alpha$
forest demands new stringent criteria to ensure the identification
of the O~VI doublet. We used simulated spectra to
determine the statistical significance of lines indentified
in the Lyman-$\alpha$ forest.
We found 12 O~VI doublets and 9 are expected to be real. Six
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 is
$\left
A rough estimate of the cosmological mass density of O~VI is carried out.
If we assume that O~VI lines are linear, we get a lower limit of
$\Omega(OVI) \geq 3 \times 10^{-9}
\, h^{-1}_{100} $.
Since O $>$ O~VI, if
the mean metal abundance were below 0.002 solar, then the accompanying
Hydrogen and Helium
would account for all baryons in the universe. We conclude that
mean abundances are above 0.002 solar, and much greater if the
gas is not highly ionized (O $\gg$ O~VI).
