HIGH-REDSHIFT METALS. I. THE DECLINE OF C IV ATz> 5.3

We present the results from our search for C iv absorption systems at redshifts z = 5.3–6.0. We have observed four z ∼ 6 QSOs with Keck/NIRSPEC in echelle mode. The data are the most sensitive yet taken to search for C iv at these redshifts, being 50% complete at column densities log N C iv ≈ 13.4(cm −2 ). We find no clear C iv systems in any of the four sight lines. Taking into account our completeness, this translates into a decline in the number density of C iv absorbers in the range 13.2 < log N C iv < 15.0 of at least a factor ∼4.1 (95% confidence) from z ∼ 2–4.5, over which the number density is relatively constant. We use our lack of detections, along with results from previous studies, to set limits on the slope and normalization of the column density distribution at z = 5.3–6.0. The rapid evolution of C iv at these redshifts suggests that the decrease in the number density may largely be due to ionization effects, in which case many of the metals in the z ∼ 4.5 intergalactic medium (IGM) could already be in place at z ∼ 5.3, but in lower ionization states. The lack of weak systems in our data, combined with the presence of strong C iv absorbers along at least one other sight line, further suggests that there may be large-scale variations in the enrichment and/or ionization state of the z ∼ 6 IGM. Alternatively, the known C iv absorbers at these redshifts may not reside in the general IGM, but may be associated with rare, UV-bright star-forming galaxies.