A downturn in intergalactic CIV 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 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.
