Heating of the Intergalactic Medium by Hydrogen Reionization

During reionization, the intergalactic medium is heated impulsively by supersonic ionization fronts (I-fronts). The peak gas temperatures behind the I-fronts, T-reion, are a key uncertainty in models of the thermal history after reionization. Here we use high-resolution radiative transfer (RT) simulations to study the parameter space of T-reion. We show that T-reion 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 T-reion by measuring I-front speeds in cosmological RT simulations. We find that the post-I-front temperatures evolve toward hotter values as reionization progresses. Temperatures of T-reion = 17,000-22,000 K are typical during the first half of reionization, but T-reion = 25,000-30,000 K may be achieved near the end of this process if I-front speeds reach similar to 10(4) km s(-1), as found in our simulations. Shorter reionization epochs lead to hotter T-reion. We discuss implications for z > 5 Ly alpha 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 T-reion on the I-front speed.