2002ApJ...574L.111T

The expansion of the universe leads to a rapid drop in the hydrogen Lyα effective optical depth of the intergalactic medium (IGM), τ{d1}_eff∝(1+z)^3.8, between redshifts 4 and 3. Measurements of the temperature evolution of the IGM and of the He II opacity both suggest that He II reionizes in this redshift range. We use hydrodynamical simulations to show that the observed temperature increase associated with He II reionization leads to a relatively sudden decrease in τ{d1}_eff around the reionization epoch of ~10%. We find clear evidence for such a feature in the evolution of τ{d1}_eff determined from a sample of ∼1100 quasars obtained from the Sloan Digital Sky Survey. He II reionization starts at redshift ~3.4 and lasts for Δz~0.4. The increase in the IGM temperature also explains the widths of hydrogen absorption lines as measured in high-resolution spectra.