Frequency of Oxygen VI in Intervening QSO Absorption Systems

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Session 70 -- Active Galaxies and Intergalactic Medium
Oral presentation, Tuesday, 10, 1995, 2:00pm - 3:30pm

[70.03] Frequency of Oxygen VI in Intervening QSO Absorption Systems

Scott Burles, David Tytler (CASS, University of California, San Diego)

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 $\left = 1.0 \pm0.6$ at a mean absorption redshift of $z_{ave} = 0.9$. For comparable $W_r$ the density of O~VI absorbers is similar to Mg~II (Tytler et al 1986; Steidel \& Sargent 1992) and C~IV absorbers (Sargent et al 1988; Bahcall et al 1993). We searched for other common ions in the O~VI absorption systems. Out of 8 O~VI absorption systems in which C~IV is also found, C~IV is stronger in all except $z_{abs}$=1.0828 towards PG1206+459 which we believe is collisionally ionized.

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).

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