Cosmological Reionization by Stellar Sources

I use cosmological simulations that incorporate a physically motivated approximation to three-dimensional radiative transfer that recovers correct asymptotic ionization front propagation speeds for some cosmologically relevant density distributions to investigate the process of the reionization of the universe by ionizing radiation from protogalaxies. Reionization proceeds in three stages and occupies a large redshift range from z~15 until z~5. During the first, ``preoverlap'' stage, H II regions gradually expand into the low-density intergalactic medium, leaving behind neutral high-density protrusions. During the second, ``overlap'' stage, that occurs in about 10% of the Hubble time, H II regions merge and the ionizing background rises by a large factor. During the third, ``postoverlap'' stage, remaining high-density regions are being gradually ionized as the required ionizing photons are being produced. Residual fluctuations in the ionizing background reach significant (more than 10%) levels for the Lyα forest absorption systems with column densities above 10¹⁴-10¹⁵ cm^-2 at z=3-4.