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 z(em) > 5.7. We detect three strong C IV doublets at z(abs) = 5.7-5.8, two low-ionization systems at z(abs) > 5 and numerous Mg II absorbers at z(abs) = 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 Omega(CIV) = (4.4 +/- 2.6) x 10(-9) which implies a drop by a factor of similar to 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 similar to 300 Myr; such a build-up could reflect the accumulation of metals associated with the rising levels of star formation activity from z similar to 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 Omega(CIV) we derive is typical of the IGM at large, it would imply a metallicity Z(IGM) greater than or similar to 10(-4) Z(circle dot). 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.