The CivMass Density of the Universe at Redshift 5

In order to search for metals in the Lyα forest at redshifts z abs > 4, we have obtained spectra of high signal-to-noise ratio and moderately high resolution of three QSOs at z em > 5.4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N (C ) = 12.50-13.98 over a total redshift path Δ X = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C 3+ ions Ω = (4.3 ± 2.5) × 10 -8 (90% confidence limits) for absorption systems with log N (C ) ≥ 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Ω is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time that stretches from ~1.25 to ~4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at z ≫ 5, perhaps by the sources responsible for its reionization. Alternatively, the C IV systems we see may be associated with outflows from massive star-forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Lyα forest of lower density than those probed up to now.