High Redshift Metals I.: The Decline of C IV at z > 5.3

TL;DR

This study searches for C IV metal absorption in the intergalactic medium at redshifts 5.3-6.0 using Keck/NIRSPEC, finding a significant decline in C IV systems compared to lower redshifts, implying rapid ionization evolution or large-scale variation.

Contribution

First sensitive search for C IV at z>5.3, establishing a decline in its number density and setting limits on the column density distribution at these high redshifts.

Findings

No C IV systems detected in four sightlines at z=5.3-6.0.

At least a 4.4-fold decline in C IV absorber density compared to z~2-4.5.

Indications of large-scale variation or association with star-forming galaxies.

Abstract

We present the results from our search for C IV in the intergalactic medium 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_{CIV})=13.4. We find no C IV systems in any of the four sightlines. Taking into account our completeness, this translates into a decline in the number density of C IV absorbers in the range 13.0 < log(N_{CIV}) < 15.0 of at least a factor 4.4 (95% confidence) from z~2-4.5, where the number density is relatively constant. We use our lack of detections 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 IGM could already be in place at z~5.3, but in a lower ionization state. The lack of weak systems in our data, combined with the presence of strong C IV absorbers along at least one other sightline, further suggests that there may be large-scale variations in the enrichment and/or ionization state of the z~6 IGM, or that C IV absorbers at these redshifts are associated with rare, UV-bright star-forming galaxies.