A downturn in intergalactic CIV as redshift 6 is approached

TL;DR

This study presents the first evidence of a decline in intergalactic C IV metal density at redshifts above 5, indicating rapid metal build-up and changes in ionisation in the early universe.

Contribution

It provides the largest survey of intergalactic metals at z > 5 and reports the first detection of a decrease in C IV density at high redshift.

Findings

Omega(C IV) drops by a factor of 3.5 at z > 5

Number of C IV doublets is lower than expected for non-evolving absorbers

Metallicity of the IGM is estimated at 10^(-4) Z(Sun)

Abstract

We present the results of the largest survey to date for intergalactic metals at redshifts z > 5, using near-IR spectra of nine QSOs with emission redshifts z(em) > 5.7. We find, for the first time, a change in the comoving mass density of C IV ions as we look back to redshifts z > 5. At a mean <z> = 5.76, we deduce Omega(C IV)=(4.4+/-2.6)x10^(-9) which implies a drop by a factor of about 3.5 compared to the value at z < 4.7, after accounting for the differing sensitivities of different surveys. The observed number of C IV doublets is also lower by a similar factor, compared to expectations for a non-evolving column density distribution of absorbers. These results point to a rapid build-up of intergalactic C IV over a period of only 300 Myr; such a build-up could reflect the accumulation of metals associated with the rising levels of star formation activity from z = 9 indicated by galaxy counts, and/or an increasing degree of ionisation of the intergalactic medium (IGM), following the overlap of ionisation fronts from star-forming regions. If the value of Omega(C IV) we derive is typical of the IGM at large, it would imply a metallicity Z(IGM) = 10^(-4) Z(Sun). The early-type stars responsible for synthesising these metals would have emitted only about one Lyman continuum photon per baryon prior to z = 5.8; such a background is insufficient to keep the IGM ionised and we speculate on possible factors which could make up the required shortfall.