Heating of the Intergalactic Medium by Hydrogen Reionization
Abstract
During reionization, the intergalactic medium is heated impulsively by supersonic ionization fronts (I-fronts). The peak gas temperatures behind the I-fronts, Treion, are a key uncertainty in models of the thermal history after reionization. Here we use high-resolution radiative transfer simulations to study the parameter space of Treion. We show that Treion is only mildly sensitive to the spectrum of incident radiation over most of the parameter space, with temperatures set primarily by I-front speeds. We also explore what current models of reionization predict for Treion by measuring I-front speeds in cosmological radiative transfer simulations. We find that the post-I-front temperatures evolve toward hotter values as reionization progresses. Temperatures of Treion = 17,000-22,000 K are typical during the first half of reionization, but Treion = 25,000 - 30,000 K may be achieved near the end of this process if I-front speeds reach 104 km/s as found in our simulations. Shorter reionization epochs lead to hotter Treion. We discuss implications for z>5 Lyα forest observations, which potentially include sight lines through hot, recently reionized patches of the Universe. Interpolation tables from our parameter space study are made publicly available, along with a simple fit for the dependence of Treion on the I-front speed.