The distribution of metallicity in the intergalactic medium at z ∼ 2.5:: OVI and CIV absorption in the spectra of seven QSOs

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) similar to 2.5) contains lines with N-H i greater than or equal to 10(13.6), encompassing similar to50% of all baryons at z similar to 2.5. Our survey accounts for approximate to98.8% of the C IV mass and approximate to90% 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]) approximate to -2.85 and sigma = 0.75 dex. We discuss the sensitivity of these results to the assumed form of the ionizing UV radiation field. Some similar to60%-70% of lines in the Lyalpha forest are enriched to observable levels of [O, C/H] greater than or similar to -3.5, while the remaining similar to30% 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] greater than or similar to -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 similar to 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.