We have studied the abundance of oxygen in the IGM by analyzing O VI, C IV, Si IV, and H I pixel optical depths derived from a set of high-quality VLT and Keck spectra of 17 QSOs at 2.1 z 3.6. Comparing ratios τO VI/τC IV(τC IV) to those in realistic, synthetic spectra drawn from a hydrodynamical simulation and comparing to existing constraints on [Si/C] places strong constraints on the ultraviolet background (UVB) model using weak priors on allowed values of [Si/O]: for example, a quasar-only background yields [ Si/O ] ≈ 1.4, which is highly inconsistent with the [ Si/O ] ≈ 0 expected from nucleosynthetic yields and with observations of metal-poor stars. Assuming a fiducial quasar+galaxy UVB consistent with these constraints yields a primary result that [ O/C ] = 0.66 ± 0.06 ± 0.2; this result pertains to gas with overdensity δ 2. Consistent results are obtained by similarly comparing τO VI/τH I(τH I) and τO VI/τSi IV(τSi IV) to simulation values, and also by directly ionization-correcting τO VI/τH I as a function of τH I into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-τH I samples are inconsistent, suggesting either density dependence of [O/C] or—more likely—prevalence of collisionally ionized gas at high density.
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