Constraints on Reionization from the Thermal History of the Intergalactic Medium

Abstract

The temperature of the diffuse, photoheated intergalactic medium (IGM) depends on its reionization history
\nbecause the thermal timescales are long. The widths of the hydrogen Lya absorption lines seen in the spectra
\nof distant quasars that arise in the IGM can be used to determine its temperature. We use a wavelet analysis of
\nthe Lya forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths
\nbetween redshifts z ≈ 3.5 and 3.0, which we associate with entropy injection resulting from the reionization of
\nHe ii. The subsequent falloff in temperature after z ≈ 3.5 is consistent with a thermal evolution dominated by
\nadiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high
\ntemperatures inferred from the line widths before He ii reionization imply that hydrogen reionization occurred
\nbelow redshift z p 9.