After the era of recombination, roughly 360 000 years after the big bang, 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, around 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.
I will present results from simulations using different source models and discuss briefly different measures to quantify the sizes of the emerging H II regions. I also present a new version of the widely used radiative transfer code C2-Ray, which is capable of handling the high energetic ionizing radiation produced by quasars. Using this new version of the radiative transfer code, we studied whether 21cm experiments could detect the signature of a quasar during the EoR. 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.
