A downturn in intergalactic C IV as redshift 6 is approached

We present the results of the largest survey to date for intergalactic metals at redshifts z > 5 , using near-IR spectra of nine quasi-stellar objects with emission redshifts zem > 5.7 . We detect three strong C iv doublets at zabs= 5.7–5.8 , two low-ionization systems at zabs > 5 and numerous Mg ii absorbers at zabs= 2.5–2.8 . 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 ΩC iv= (4.4 ± 2.6) × 10−9 which implies a drop by a factor of ∼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 ionization of the intergalactic medium (IGM), following the overlap of ionization fronts from star-forming regions. If the value of ΩC iv we derive is typical of the IGM at large, it would imply a metallicity ZIGM≳ 10−4 Z⊙ . The early-type stars responsible for synthesizing 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 ionized and we speculate on possible factors which could make up the required shortfall