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Published June 20, 2009 | Published
Journal Article Open

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

Abstract

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.

Additional Information

© 2009. The American Astronomical Society. Received 2008 December 11; accepted 2009 April 3; published 2009 May 27. The authors thank Emma Ryan-Weber and Max Pettini for many stimulating conversations over the course of this work. We also thank Martin Haehnelt, Bob Carswell, and Rob Simcoe for providing comments on the first draft of this paper, as well as the anonymous referee for several helpful suggestions. The authors wish to recognize the very significant cultural role that the summit of Mauna Kea has always had within the indigenous Hawaiian community.We are most fortunate to have the opportunity to conduct observations from this mountain. G.B. was supported by the Kavli Institute for Cosmology at the Institute of Astronomy in Cambridge. M.R. was supported by the National Science Foundation through grant AST-0506845. W.S. was supported through grant AST-0606868.

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