2004ApJ...606...92S

We present a direct measurement of the metallicity distribution function for the high-redshift intergalactic medium (IGM). We determine the shape of this function using survival statistics, which incorporate both detections and nondetections of O VI and C IV absorption, associated with H I lines in high-resolution quasar spectra. The O VI sample (taken from seven QSOs at z_abs∼2.5) contains lines with N_HI_≥10^13.6, encompassing ∼50% of all baryons at z∼2.5. Our survey accounts for ~98.8% of the C IV mass and ~90% of the O VI mass in the universe at this epoch. We find a median intergalactic abundance of [O,C/H]=-2.82; the differential abundance distribution is approximately lognormal with mean [C,O/H]~-2.85 and σ=0.75 dex. We discuss the sensitivity of these results to the assumed form of the ionizing UV radiation field. Some ∼60%-70% of lines in the Lyα forest are enriched to observable levels of [O,C/H]≳-3.5, while the remaining ∼30% of the lines have even lower abundances. Thus we have not detected a universal metallicity floor, as has been suggested for some Population III enrichment scenarios. In fact, we argue that the bulk of the intergalactic metals formed later than the first stars that are thought to have triggered reionization. We do not observe a strong trend of decreasing metallicity toward the lower density IGM, at least within regions that would be characterized as filaments in numerical simulations. However, an [O/H] enhancement may be present at somewhat high densities. We estimate that roughly half of all baryons at these redshifts have been enriched to [O,C/H]≳-3.5. Using a simple ``closed box'' model for the metallicity evolution of the IGM, we estimate the chemical yield of galaxies formed prior to z∼2.5, finding that the typical galaxy recycled 0.1%-0.4% of its mass back into the IGM as heavy elements in the first 3 Gyr after the big bang.