Reionization and thermal evolution of a photoionized intergalactic medium

In most models of the reionization of the intergalactic medium, sources of UV radiation create ionized regions around them which expand until they overlap. The gas is impulsively heated when it enters one of these ionized regions. We discuss the various physical effects that determine the temperature of the gas after the passage of the ionization front. Sources with different spectra may heat the surrounding gas to different temperatures, creating large-scale inhomogeneities of the gas temperature in the Universe. The presence of helium is particularly important, since it allows the post-reionization temperature to reach values as high as $$\sim 5\times10^4 \ {\rm K}$$. Subsequent adiabatic heating as gas collapses into haloes can raise the temperature even more, explaining large b-parameters in $${\rm Ly} \ \alpha$$ clouds in the minihalo model. The gas heated in this way retains memory of the initial temperature and the reheating epoch, since the thermal time-scale is long. $${\rm Ly} \ \alpha$$ absorption lines of low column density, which arise in the non-virialized outer parts of haloes with over-density less than $$\sim 10$$, could show a spatial correlation caused by variations in the initial temperature. We also investigate the modifications of the proximity effect produced by the different equilibrium temperature of the gas in the vicinity of a quasar. This new proximity effect could also cause the lines close to the emitting quasar to have a different distribution of b-parameters.