Metallicity of the intergalactic medium using pixel statistics: IV. Oxygen

TL;DR

This study measures oxygen abundance in the intergalactic medium using pixel optical depths from high-quality spectra, constraining the ultraviolet background model and deriving a key oxygen-to-carbon ratio with implications for cosmic chemical enrichment.

Contribution

It introduces a novel pixel statistical method to determine oxygen abundance and UV background constraints, improving understanding of IGM metallicity and ionization conditions.

Findings

[O/C] = 0.66 +/- 0.06 +/- 0.2 in the IGM

Quasar-only UVB models are inconsistent with observed abundance ratios

No evidence for evolution of oxygen abundance over studied redshift range

Abstract

We have studied the abundance of oxygen in the IGM by analyzing OVI, CIV, SiIV, and HI pixel optical depths derived from a set of high-quality VLT and Keck spectra of 17 QSOs at 2.1 < z < 3.6. Comparing OVI and CIV optical depth ratios 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, 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 >~ 0.2. Consistent results are obtained by similarly comparing OVI to HI or OVI to SiIV optical depth ratios to simulation values, and also by directly ionization-correcting OVI optical depths as function of HI optical depths into [O/H] as a function of density. Subdividing the sample reveals no evidence for evolution, but low- and high-density samples are inconsistent, suggesting either density-dependence of [O/C] or -- more likely -- prevalence of collisionally-ionized gas at high density.