Limiting the Hydrogen reionization redshift from the temperature evolution of the IGM (Theuns et al 2002)

Shown is the temperature evolution of the IGM above redshift 3.4. The solid curves indicate the evolution of the temperature at the mean density for various HI reionization redshifts z_H, as indicated. The post-hydrogen reionization temperature is assumed to be 60,000K. The HeII photo-ionization rate is adjusted so that he HeII abundance is 0.1 at redshift 3.5. If He is mostly singly ionized at redshift 3.5, then the rapid decrease in the IGM temperature after reionization places an upper limit of 9 on the redshift of hydrogen reionization