Models of the thermal evolution of the intergalactic medium after reionization

Recent years have brought more precise temperature measurements of the low-density intergalactic medium (IGM). These new measurements constrain the processes that heated the IGM, such as the reionization of H i and of He ii. We present a semi-analytical model for the thermal history of the IGM that follows the photoheating history of primordial gas. Our model adopts a multizone approach that, compared to previous models, more accurately captures the inhomogeneous heating and cooling of the IGM during patchy reionization. We compare our model with recent temperature measurements spanning z = 1.6–4.8, finding that these measurements are consistent with scenarios in which the He ii was reionized at z = 3–4 by quasars. Significantly longer duration or higher redshift He ii reionization scenarios are ruled out by the measurements. For hydrogen reionization, we find that only low-redshift and high-temperature scenarios are excluded. For example, a model in which the IGM was heated to 30 000 K when an ionization front passed, and with hydrogen reionization occurring over 6 3.