METALLICITY OF THE INTERGALACTIC MEDIUM USING PIXEL STATISTICS. IV. OXYGEN

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 VLTand Keck spectra of 17 QSOs at 2.1 less than or similar to z less than or similar to 3.6. Comparing ratios tau O (VI)/tau C (IV)(tau C 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] approximate to 1.4, which is highly inconsistent with the [Si/O] approximate to 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 delta greater than or similar to 2. Consistent results are obtained by similarly comparing tau O (VI)/tau H-I (tau H-I) and tau O-VI/tau Si-IV (tau Si-IV) to simulation values, and also by directly ionization-correcting tau O-VI/tau H-I as a function of tau H-I into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-tau H-I samples are inconsistent, suggesting either density dependence of [O/C] or-more likely-prevalence of collisionally ionized gas at high density.