ABSTRACT The temperature of the diffuse, photoheated intergalactic medium (IGM) depends on its reionization history because the thermal timescales are long. The widths of the hydrogen Lya 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 Lya forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths between redshifts and 3.0, which we associate with entropy injection resulting from the reionization of z ≈ 3.5 He ii. The subsequent falloff in temperature after is consistent with a thermal evolution dominated by z ≈ 3.5 adiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high temperatures inferred from the line widths before He ii reionization imply that hydrogen reionization occurred below redshift . z p
