The Nature of the Ionising 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< z <5. Our approach makes use of observational estimates of the opacities of hydrogen and singly ionised helium in the intergalactic medium during this epoch. In particular, we model the reionisation of He II by sources of hard ultraviolet radiation, i.e. quasars, and infer values for our parameterisation of this population from observational estimates of the opacity of the He II Lyman-alpha forest. Next, we estimate the photoionisation rate of H I from these sources and find that their contribution to the ionising background is insufficient to account for the measured opacity of the H I Lyman-alpha forest at a redshift z 3. This motivates us to include a soft, stellar component to the ionising background to boost the hydrogen photoionisation 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 about equally to the ionising background in H I at z~ 3.