Constraints on reionization from the thermal history of the intergalactic medium

The temperature of the diffuse, photo-heated intergalactic medium (IGM) depends on its reionization history because the thermal time scales are long. The widths of the hydrogen Lyman-alpha absorption lines seen in the spectra of distant quasars that arise in the IGM can be used to determine its temperature. We use a wavelet analysis of the Lyman-alpha forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths between redshifts z=3.5 and 3.0, which we associate with entropy injection resulting from the reionization of HeII. The subsequent fall-off in temperature after z~3.5, is consistent with a thermal evolution dominated by adiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high temperatures inferred from the line widths before HeII reionization imply that hydrogen reionization occurred below redshift z=9.