  The structures produced during the epoch of reionization by the action of
radiation on neutral hydrogen are in principle different to those that arise
through gravitational growth of initially small perturbations. We explore the
difference between the two mechanisms using high resolution cosmological
radiative transfer. Our computations use a Monte Carlo code which raytraces
directly through SPH kernels without a grid, preserving the high spatial
resolution of the underlying hydrodynamic simulation. Because the properties of
the first sources of radiation are uncertain, we simulate a range of models
with different source properties and recombination physics. We examine the
morphology of the neutral hydrogren distribution and the reionization history
in these models. We find that at fixed mean neutral fraction, structures are
visually most affected by the existence of a lower limit in source luminosity,
then by galaxy mass to light ratio, and are minimally affected by changes in
the recombination rate and amplitude of mass fluctuations. We concentrate on
the autocorrelation function of the neutral hydrogen, xi_HI(r) as a basic
quantitive measure of Radiation Induced Structure (RIS.) All the models we test
exhibit a characteristic behaviour, with xi_HI(r) becoming initially linearly
antibiased with respect to the matter correlation function, reaching a minimum
bias factor b~0.5 when the universe is ~10-20% ionized. After this xi_HI(r)
increases rapidly in amplitude, overtaking the matter correlation function. It
keeps a power law shape, but flattens considerably, reaching an asymptotic
logarithmic slope of gamma ~-0.5. The growth rate of HI fluctuations is
exponentially more rapid than gravitational growth over a brief interval of
redshift Deltaz ~ 2-3.

Astrophysics