The State of the Universe at z~6
  In the context stellar reionization in the standard cold dark matter model,
we analyze observations at z~6 and are able to draw three significant
conclusions with respect to star formation and the state of the intergalactic
medium (IGM) at z~6. (1) An initial stellar mass function (IMF) more efficient,
by a factor of 10-20, in producing ionizing photons than the standard Salpeter
IMF is required at z~6. This may be achieved by having either (A) a
metal-enriched IMF with and a lower mass cutoff of >= 30Msun or (B) 2-4% of
stellar mass being Population III massive metal-free stars at z~6. While there
is no compelling physical reason or observational evidence to support (A), (B)
could be fulfilled plausibly by continued existence of some pockets of
uncontaminated, metal-free gas for star formation. (2) The volume-weighted
neutral fraction of the IGM of <f_HI>_V~ 10^-4 at z=5.8 inferred from the SDSS
observations of QSO absorption spectra provides enough information to ascertain
that reionization is basically complete with at most ~0.1-1% of IGM that is
un-ionized at z=5.8. (3) Barring some extreme evolution of the IMF, the neutral
fraction of the IGM is expected to rise quickly toward high redshift from the
point of HII bubble percolation, with the mean neutral fraction of the IGM
expected to reach 6-12% at z=6.5, 13-27% at z=7.7 and 22-38% at z=8.8.
