The nature of the ionizing background at z ≈ 2.5-5

Using radiative transfer calculations and cosmological simulations of structure formation, we study constraints that can be placed on the nature of the cosmic ultraviolet (UV) background in the redshift interval 2.5 less than or similar toz less than or similar to 5. Our approach makes use of observational estimates of the opacities of hydrogen and singly ionized helium in the intergalactic medium during this epoch. In particular, we model the reionization of He II by sources of hard ultraviolet radiation, i.e. quasars, and infer values for our parametrization of this population from observational estimates of the opacity of the He II Lyman-alpha forest. Next, we estimate the photoionization rate of H I from these sources and find that their contribution to the ionizing background is insufficient to account for the measured opacity of the H I Lyman-alpha forest at a redshift zsimilar to3. This motivates us to include a soft, stellar component to the ionizing background to boost the hydrogen photoionization rate, but which has a negligible impact on the He II opacity. In order to simultaneously match observational estimates of the H I and He II opacities, we find that galaxies and quasars must contribute approximately equally to the ionizing background in H I at zsimilar or equal to3. Moreover, our analysis requires the stellar component to rise for z>3 to compensate for the declining contribution from bright quasars at higher redshift. This inference is consistent with some observational and theoretical estimates of the evolution of the cosmic star formation rate. The increasing dominance of the stellar component towards high redshift leads to a progressive softening of the UV background, as suggested by observations of metal line absorption. In the absence of additional sources of ionizing radiation, such as mini-quasars or weak active galactic nuclei, our results, extrapolated to z>5, suggest that hydrogen reionization at zsimilar to6 mostly probably occurred through the action of stellar radiation.