Radiative effects by high-z UV radiation background: Implications for the future CMB polarization measurements

The radiative effects by high-redshift ultraviolet radiation background (UVB) heats the gas in the IGM and eliminate the neutral hydrogen and helium that dominate the cooling of the primordial gas. We investigate the role of the radiative effects for the temporal evolution of the reionization fraction by using cosmological Smooth Particle Hydrodynamics (SPH) simulations. We find that the increase of photo-ionization and photo-heating rates due to optical depth effects results in a significantly contribute to the heating of the IGM before and during the reionization. The main effect of the UV radiation spectrum on the temporal evolution of the ionization fraction is given by the value of the reionization redshift, zre, and the redshift interval, ∆z, in which the reionization is completed. We evaluate the effects of the UV radiation background on the CMB angular power spectrum taking into account different temporal evolutions of the ionization fraction. We find that for reionization models with degenerated CMB temperature anisotropy power spectra, the radiative mechanisms leave distinct signatures on the E-mode polarization power spectrum, at large scales (l < 50). We show that through E-mode CMB polarization power spectrum measurements, the Planck experiment will have the sensitivity to distinguish between different reionization histories even when they imply the same optical depth to electron scattering and degenerated CT power spectra. This work has been done in the framework of the Planck LFI activities.