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

摘要

Recent observations suggest that hydrogen reionization ends late ($z simeq6$) and proceeds quickly. We present here a new model of the meta-galacticUV/X-ray background (UVB) that is consistent with this. It adopts the mostrecent determinations of the ionizing emissivity due to stars and AGN, as wellas of the HI absorber column density distribution. Another major improvement isa new treatment of the intergalactic medium (IGM) opacity for ionizing photonsthat is able to consistently capture the transition from a neutral to anionized IGM. Previous synthesis models of the UVB, when used in simulations,yield reionization and thermal histories that are inconsistent with the assumedionizing emissivities. With our new treatment, this discrepancy is fullyresolved. In our fiducial model, galaxies leaking $lesssim 18%$ of theirLyman continuum emission drive HI reionization, while AGN drive HeIIreionization (completing at $z simeq 6.2$ and $2.8$ respectively). Due to thelimited time available for cooling between HI and HeII reionization, higher IGMtemperatures are predicted for late reionization scenarios. In our fiducialmodel, the predicted temperatures agree well with observational constraints at$z lesssim 4$, while being slightly high compared to (somewhat uncertain) dataabove that. Models with a larger contribution of AGN are instead disfavoured bythe temperature data, as well as by measurements of the HI and HeIILyman-$lpha$ forest opacities. We also present "equivalent-equilibrium"ionization/heating rates that mimic our fiducial UVB model for use insimulation codes that assume ionization equilibrium.