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

We present a systematic search for O VI (1032, 1037 Angstrom) absorption in a Keck High-Resolution Echelle Spectrometer spectrum of the z = 3.62 quasa r Q1422 + 231, with the goal of constraining the metallicity and ionization state of the low-density intergalactic medium (IGM). Comparison of C rv ab sorption measurements to models of the Ly alpha forest based on cosmologica l simulations shows that absorbers with N-HI greater than or similar to 10( 14.5) cm(-2) have a mean carbon abundance [C/H] approximate to -2.5, assumi ng a metagalactic photoionizing background with the spectral shape predicte d by Haardt and Madau in 1996. In these models, lower column density absorp tion arises in lower density gas where most C IV is photoinized to C V. The refore, O VI should be the most sensitive tracer of metallicity in Ly alpha absorbers with N-HI less than or similar to 10(14.5) cm(-2). O VI lines li e at wavelengths heavily contaminated by Lyman series absorption, so we int erpret the search results by comparing them to carefully constructed, mock Q1422 spectra drawn from a hydrodynamic simulation of a Lambda-dominated co ld 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 bl ankets the position of the doublet companion line in each case, and the tot al number of narrow lines is statistically consistent with that in zero-met allicity artificial spectra. Artificial spectra generated with the Haardt a nd Madau background and [O/H] greater than or similar to -2.5 predict too m any 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 absorp tion; matching the data requires [O/C] approximate to 0.5 and corresponding [O/H] approximate to -2.0. Taken together, the narrow-line and optical dep th 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 wh ere 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 photoioni zing background spectrum would be truncated above 4 Ryd in regions that hav e not undergone helium reionization (He II --> He III), and in this case ma tching the Q1422 data requires lower [C/H] but higher [O/H]. Taking [O/C] a pproximate to 1 as the maximum plausible overabundance of oxygen, we conclu de that helium must have been reionized through at least 50% of the volume from z similar to 3-3.6.