Simulations of Cosmic Reionization : Shapes & Sizes of H II regions around Galaxies and Quasars

After the era of recombination, roughly 360 000 years after the big bang (redshift 1100), the universe was neutral, continued to expand and eventually the first gravitationally collapsed structures capable of forming stars, formed. Observations show that approximately 1 billion years later (redshift 6), the Universe had become highly ionized. The transition from a neutral intergalactic medium to a highly ionized one, is called the epoch of Reionization (EoR). Although quasar spectra and polarization power-spectra from cosmic microwave background experiments set some time-constrains on this epoch, the details of this process are currently not known. New radio telescopes operating at low frequencies aim at measuring directly the neutral hydrogen content between redshifts 6 - 10 via the HI spin-flip line at 21cm. The interpretation of these first measurements is not going to be trivial. Therefore, simulations of the EoR are useful to test the many ill-constrained parameters such as the properties of the sources responsible for reionization. This thesis contributes to such simulations. It addresses different source models and discusses different measures to quantify their effect on the shapes and sizes of the emerging H II regions. It also presents a new version of the widely used radiative transfer code C2-Ray which is capable of handling the ionizing radiation produced by energetic sources such as quasars. Using this new version we study whether 21cm experiments could detect the signature of a quasar. We find that different size measures of ionized regions can distinguish between different source models in the simulations and that a topological measure of the ionized fraction field confirms the inside-out (i.e. overdense regions ionize first) reionization scenario. We find that the HII regions from luminous quasars may be detectable in 21cm, but that it might not be possible to distinguish them from the largest HII regions produced by clustered galaxies. At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Submitted.