High-Redshift Intergalactic C IV Abundance Measurements from the Near-Infrared Spectra of Two z ~ 6 QSOs

New measurements of the z~6 intergalactic C IV abundance are presented, using moderate-resolution IR spectra of two QSOs taken with GNIRS on Gemini-South. These data were systematically searched for high-redshift C IV absorption lines, using objective selection criteria. Comprehensive tests were performed to quantify sample incompleteness, as well as the rate of false-positive C IV identifications. The trend of constant Ω_CIV(z) observed at z~2-5 appears to continue to z~6, the highest observed redshift. The C IV sample is also consistent with the redshift-invariant form of the C IV column density distribution reported by Songaila at lower redshift, although with fairly large uncertainties due to a smaller sample size and noisier infrared data. The constant value of Ω_CIV does not necessarily imply that the IGM was infused with an early metallicity ``floor,'' but the presence of early C IV does indicate that heavy-element enrichment began <~1 Gyr after the big bang. The lack of a decline in Ω_CIV at high redshift may indicate that integrated C IV measurements are sensitive to the instantaneous rate of feedback from galaxy formation at each epoch. Alternatively, it could result from a balance in the evolution of the intergalactic gas density, ionization conditions, and heavy-element abundance over time.

Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).