The CivMass Density of the Universe at Redshift 5

In order to search for metals in the Lyman alpha forest at redshifts z > 4,we have obtained spectra of high S/N and resolution of three QSOs at z > 5.4discovered by the Sloan Digital Sky Survey. These data allow us to probe tometal enrichment of the intergalactic medium at early times with highersensitivity than previous studies. We find 16 C IV absorption systems withcolumn densities log N(C IV) = 12.50 - 13.98 over a total redshift path Delta X= 3.29. In the redshift interval z = 4.5-5.0, where our statistics are mostreliable, we deduce a comoving mass density of C IV ions Omega(C IV) = (4.3 +/-2.5) x 10(-8) (90% confidence limits) for absorption systems with log N(C IV) >13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Omega(CIV) is entirely consistent with those measured at z < 4; we confirm the earlierfinding by Songaila (2001) that neither the column density distribution of C IVabsorbers nor its integral show significant redshift evolution over a period oftime which stretches from 1.25 to 4.5 Gyr after the big bang. This somewhatsurprising conclusion may be an indication that the intergalactic medium wasenriched in metals at redshifts much greater than 5, perhaps by the sourcesresponsible for its reionization. Alternatively, the C IV systems we see may beassociated with outflows from massive star-forming galaxies at later times,while the truly intergalactic metals may reside in regions of the Lyman alphaforest of lower density than those probed up to now.Comment: 24 pages including 5 figures. Accepted for publication in the Astrophysical Journal (September 10, 2003 issue