Constraints on the universal C IV mass density at z ∼ 6 from early infrared spectra obtained with the Magellan fire spectrograph

We present a new determination of the intergalactic C IV mass density at 4.3 < z < 6.3. Our constraints are derived from high signal-to-noise spectra of seven quasars at z > 5.8 obtained with the newly commissioned Folded-Port Infrared Echellette (FIRE) spectrograph on the Magellan Baade telescope, coupled with six observations of northern objects taken from the literature. We confirm the presence of a downturn in the C IV abundance at 〈z〉 = 5.66 by a factor of 4.1 relative to its value at 〈z〉 = 4.96, as measured in the same sight lines. In the FIRE sample, a strong system previously reported in the literature as C IV at z = 5.82 is re-identified as Mg II at z = 2.78, leading to a substantial downward revision in for these prior studies. Additionally, we confirm the presence of at least two systems with low-ionization C II, Si II, and O I absorption but relatively weak signal from C IV. The latter systems may be of interest if the downward trend in at high redshift is driven in part by ionization effects.