Consistent modelling of the meta-galactic UV background and the thermal/ionization history of the intergalactic medium

Recent observations suggest that hydrogen reionization ends late (z similar or equal to 6) and proceeds quickly. We present here a new model of the meta-galactic ultraviolet/X-ray background (UVB) that is consistent with this. It adopts the most recent determinations of the ionizing emissivity due to stars and active galactic nuclei (AGN), as well as of the HI absorber column density distribution. Another major improvement is a newtreatment of the intergalactic medium (IGM) opacity for ionizing photons that is able to consistently capture the transition from a neutral to an ionized IGM. Previous synthesis models of the UVB, when used in simulations, yield reionization and thermal histories that are inconsistent with the assumed ionizing emissivities. With our new treatment, this discrepancy is fully resolved. In our fiducial model, galaxies leaking less than or similar to 18 per cent of their Lyman continuum emission drive HI reionization, while AGN drive He II reionization (completing at z similar or equal to 6.2 and 2.8, respectively). Due to the limited time available for cooling between HI and He II reionization, higher IGM temperatures are predicted for late reionization scenarios. In our fiducial model, the predicted temperatures agree well with observational constraints at z less than or similar to 4, while being slightly high compared to (somewhat uncertain) data above that. Models with a larger contribution of AGN are instead disfavoured by the temperature data, as well as by measurements of the HI and He II Lyman-alpha forest opacities. We also present 'equivalent-equilibrium' ionization/heating rates that mimic our fiducial UVB model for use in simulation codes that assume ionization equilibrium.