The Shocking Truth: The small contribution to hydrogen reionization from
  gravitational infall
  It is commonly thought that stars are responsible for reionizing the
Universe. However, deep searches for star-forming galaxies during the epoch of
reionization have not yet found sufficient galaxies to provide the necessary
ionising flux. Fast accretion shocks associated with gravitational infall of
baryons during the formation of galaxies have recently been proposed as an
alternative method of generating the required ionising photons. In this Letter
we assess the contribution to hydrogen reionization from shocked gas associated
with gravitational infall. We find that shocks can ionize no more than a few
percent of the cosmic hydrogen by z~6. However, the small fraction of ionizing
radiation produced by fast accretion shocks would be significantly more biased
than that associated with stars, leading to a modification of the luminosity
weighted source clustering by ~10%. This modification of the bias may be
measurable with future precision experiments utilising the redshifted 21cm line
to study the distribution of hydrogen during the reionization era.
