Dark-ages reionization and galaxy formation simulation - XVI. The thermal memory of reionization

Intergalactic medium temperature is a powerful probe of the epoch of reionization, as information is retained long after reionization itself. However, mean temperatures are highly degenerate with the timing of reionization, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterize how different thermal statistics of the intergalactic medium can be used to constrain reionization. Temperature is highly correlated with redshift of reionization for a period of time after the gas is heated. However as the gas cools, thermal memory of reionization is lost, and a power-law temperature-density relation is formed, T = T-0(1 + delta)(1 -gamma) with gamma approximate to 1.5. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionization likely finished at z(reion) = 6.8(-0.8)(+0.5) with a soft spectral slope of alpha = 2.8(-1.0)(+1.2). By restricting spectral slope to the range [0.5, 2.5] motivated by population II synthesis models, reionization timing is further constrained to z(reion) = 6.9(-0.5)(+0.4). We find that, in the future, the degeneracies between reionization timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history.