High‐Redshift Intergalactic CivAbundance Measurements from the Near‐Infrared Spectra of Twoz∼ 6 QSOs

Abstract

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(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 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 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.