Constraints on the Universal CIV Mass Density at z~6 from Early IR Spectra Obtained with the Magellan FIRE Spectrograph

TL;DR

This study measures the intergalactic CIV mass density at redshifts 4.3 to 6.3 using high-quality spectra from the FIRE spectrograph, revealing a significant decline at z~6 and revising previous estimates due to re-identified systems.

Contribution

First measurement of CIV mass density at z~6 using FIRE spectra, confirming a decline and correcting prior misidentifications of absorption systems.

Findings

CIV abundance decreases by a factor of 4.1 from z~4.96 to 5.66

Re-identification of a CIV system as MgII at lower redshift

Detection of low-ionization systems with weak CIV signals

Abstract

We present a new determination of the intergalactic CIV 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 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 CIV abundance at <z>=5.66 by a factor of 4.1 relative to its value at <z>=4.96, as measured in the same sightlines. In the FIRE sample, a strong system previously reported in the literature as CIV at z=5.82 is re-identified as MgII at z=2.78, leading to a substantial downward revision in $\Omega_{CIV}$ for these prior studies. Additionally we confirm the presence of at least two systems with low-ionization CII, SiII, and OI absorption but relatively weak signal from CIV. The latter systems systems may be of interest if the downward trend in $\Omega_{CIV}$ at high redshift is driven in part by ionization effects.