Confronting Cosmological Simulations with Observations of Intergalactic Metals

Using the statistics of pixel optical depths, we compare H I, C IV, and C III absorption in a set of six high-quality z ~ 3-4 quasar absorption spectra to that in spectra drawn from two different state-of-the-art cosmological simulations that include galactic outflows. We find that the simulations predict far too little C IV absorption, unless the UV background is extremely soft, and always predict far too small C III/C IV ratios. We note, however, that much of the enriched gas is in a phase ( T ~ 10 5 -10 7 K, ρ/⟨ρ⟩ ~ 0.1-10, Z ≳ 0.1 Z ☉ ) that should cool by metal line emission—which was not included in our simulations. When the effect of cooling is modeled, the predicted C IV absorption increases substantially, but the C III/C IV ratios are still far too small because the density of the enriched gas is too low. Finally, we find that the predicted metal distribution is much too inhomogeneous to reproduce the observed probability distribution of C IV absorption. These findings suggest that strong z ≲ 6 winds cannot fully explain the observed enrichment and that an additional (perhaps higher z ) contribution is required.