Cosmological Reionization by Stellar Sources

Abstract.

I use cosmological simulations that incorporate an approximation of three-dimensional radiative transfer to investigate the process of the reionization of the universe by ionizing radiation from proto-galaxies. Reionization proceeds in three stages and occupies a large redshift range from $z \sim$ 15 until $z \sim$ 5. During the first, “re-overlap” stage, H II regions gradually expand into the low density IGM, 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, “post-overlap” 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 Lyman-alpha forest absorption systems with column densities above 10¹⁴-10¹⁵ cm^-2 at z = 3 to 4.