Reionisation scenarios and the temperature of the IGM

Abstract

We examine the temperature structure of the IGM due to the passage of individual ionisation fronts using a radiative transfer (RT) code coupled to a particle-mesh (PM) N-body code. Multiple simulations were performed with different spectra of ionising radiation: a power law (goes as nu^{-0.5}), miniquasar, starburst, and a time-varying spectrum that evolves from a starburst spectrum to a power law. The RT is sufficiently resolved in time and space to correctly model both the ionisation state and the temperature across the ionisation front. We find the post-ionisation temperature of the reionised intergalactic medium (IGM) is sensitive to the spectrum of the source of ionising radiation, which may be used to place strong constraints on the nature of the sources of reionisation. Radiative transfer effects also produce large fluctuations in the HeII to HI number density ratio eta. The spread in values is smaller than measured, except for the time-varying spectrum. For this case, the spread evolves as the spectral nature of the ionising background changes. Large values for eta are found in partially ionised HeII as the power-law spectrum begins to dominate the starburst, suggesting that the large eta values measured may be indicating the onset of the HeII reionisation epoch.