  We have studied the IGM reionization process in its full cosmological context
including structure evolution and a realistic galaxy population. We have used a
combination of high-resolution N-body simulations (to describe the dark matter
and diffuse gas component), a semi-analytic model of galaxy formation (to track
the evolution of the sources of ionizing radiation) and the Monte Carlo
radiative transfer code CRASH (to follow the propagation of ionizing photons
into the IGM). The process has been followed in the largest volume ever used
for this kind of study, a field region of the universe with a comoving length
of L~20/h Mpc, embedded in a much larger cosmological simulation. To assess the
effect of environment on the reionization process, the same radiative transfer
simulations have been performed on a 10/h Mpc comoving box, centered on a
clustered region. We find that, to account for the all ionizing radiation,
objects with total masses of M~10^9 Msun must be resolved. In this case, the
simulated stellar population produces a volume averaged ionization fraction
x_v=0.999 by z~8, consistent with observations without requiring any additional
sources of ionization. We also find that environment substantially affects the
reionization process. In fact, although the simulated proto-cluster occupies a
smaller volume and produces a higher number of ionizing photons, it gets
totally ionized later. This is because high density regions, which are more
common in the proto-cluster, are difficult to ionize because of their high
recombination rates.
