2008ApJ...689..851A

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 τ_OVI_/τCIV_(τCIV_) 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 τ_OVI_/τHI_(τHI_) and τ_OVI_/τSiIV_(τSiIV_) to simulation values, and also by directly ionization-correcting τ_OVI_/τHI_ as a function of τ_HI_into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-τ_HI_ samples are inconsistent, suggesting either density dependence of [O/C] or–more likely–prevalence of collisionally ionized gas at high density.