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Constraining the metallicity of the low-density Ly alpha forest using O VI absorption

We present a systematic search for O VI (1032, 1037 Å) absorption in a Keck High-Resolution Echelle Spectrometer spectrum of the z = 3.62 quasar Q1422+231, with the goal of constraining the metallicity and ionization state of the low-density intergalactic medium (IGM). Comparison of C IV absorption measurements to models of the Lyα forest based on cosmological simulations shows that absorbers with NH I 1014.5 cm-2 have a mean carbon abundance [C/H] -2.5, assuming a metagalactic photoionizing background with the spectral shape predicted by Haardt and Madau in 1996. In these models, lower column density absorption arises in lower density gas where most C IV is photoionized to C V. Therefore, O VI should be the most sensitive tracer of metallicity in Lyα absorbers with NH I 1014.5 cm-2. O VI lines lie at wavelengths heavily contaminated by Lyman series absorption, so we interpret the search results by comparing them to carefully constructed, mock Q1422 spectra drawn from a hydrodynamic simulation of a Λ-dominated cold dark matter model. A search for deep, narrow absorption features yields only a few candidate O VI lines in the spectrum of Q1422. H I absorption blankets the position of the doublet companion line in each case, and the total number of narrow lines is statistically consistent with that in zero-metallicity artificial spectra. Artificial spectra generated with the Haardt and Madau background and [O/H] -2.5 predict too many narrow lines and are statistically inconsistent with the data. We also search for O VI associated with C IV systems, using the optical depth ratio technique of Songaila. With this method we do find significant O VI absorption; matching the data requires [O/C] 0.5 and corresponding [O/H] -2.0. Taken together, the narrow-line and optical depth ratio results imply that (1) the metallicity in the low-density regions of the IGM is at least a factor of 3 below that in the overdense regions where C IV absorption is detectable, and (2) oxygen is overabundant in the C IV regions, consistent with the predictions of Type II supernova-enrichment models and the observed abundance pattern in old halo stars. The photoionizing background spectrum would be truncated above 4 Ryd in regions that have not undergone helium reionization (He II → He III), and in this case matching the Q1422 data requires lower [C/H] but higher [O/H]. Taking [O/C] 1 as the maximum plausible overabundance of oxygen, we conclude that helium must have been reionized through at least 50% of the volume from z ~ 3-3.6.