2021ApJ...906..124Z

The epoch of reionization (EoR) marks the end of the Cosmic Dawn and the beginning of large-scale structure formation in the universe. The impulsive ionization fronts (I-fronts) heat and ionize the gas within the reionization bubbles in the intergalactic medium (IGM). The temperature during this process is a key yet uncertain ingredient in current models. Typically, reionization simulations assume that all baryonic species are in instantaneous thermal equilibrium with each other during the passage of an I-front. Here we present a new model of the temperature evolution for the ionization front by studying nonequilibrium effects. In particular, we include the energy transfer between major baryon species (e^–, H I, H II, He I, and He II) and investigate their impacts on the post-ionization front temperature T_re. For a better step-size control when solving the stiff equations, we implement an implicit method and construct an energy transfer rate matrix. We find that the assumption of equilibration is valid for a nonrelativistic ionization front (speed less than 10⁹cms^–1), but deviations from equilibrium occur for faster fronts. The post-front temperature T_re is lower by up to 19.7% (at 3×10⁹cm s^–1) or 30.8% (at 10¹⁰ cm s^–1) relative to the equilibrium case.