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 z abs > 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.