At epochs corresponding to z ~ 1000, the IGM is expected to recombine
and remain neutral until sources of radiation develop that are capable of
reionizing it. The detection of transmitted flux shortward of the
Ly
wavelength in the spectra of sources at z ~ 5 implies that the hydrogen
component of this IGM was ionized at even higher redshifts.
There is some evidence that the double reionization of helium may have
occurred later, but this is still controversial.
It appears then that substantial sources of ultraviolet photons were already
present when the Universe was less than 7% of its current age, perhaps
quasars and/or young star-forming galaxies:
an episode of pre-galactic star formation may provide a possible explanation
for the widespread existence of heavy elements (like carbon, oxygen, and
silicon) in the IGM, while the integrated radiation emitted from
quasars is likely responsible for the reionization of intergalactic helium.
Establishing the epoch of reionization and reheating is crucial for
determining its impact on several key cosmological issues, from
the role reionization plays in allowing protogalactic objects to cool and
make stars, to determining the small-scale structure in the temperature
fluctuations of the cosmic background radiation. Conversely, probing the
reionization epoch may provide a means for constraining competing models for
the formation of cosmic structures, and of detecting the onset of the first
generation of stars, galaxies, and black holes in the Universe.