Detection of HeII reionization, due to its effect on the Hydrogen Lyman-alpha opacity (Theuns et al 2002).

Shown is the deviation of the effective optical depth in the Lyman-alpha forest from a power-law evolution, tau/(1+z)^{3.8}, for the Sloan Digitial Sky Survey data smoothed on 3000 kms (symbols with error bars) and a hydrodynamical simulation of a LambdaCDM model, in which HeII reionization starts at z=3.4 (hashed region). The errors in the SDSS data are determined from bootstrap e-sampling the individual spectra. The hashed region for the simulation delineates the 20 and 80 per centile of bootstrap re-sampled mock samples. The temperature increase associated with HeII reionization causes tau to drop below the power-law evolution in the simulation. This characterictic dip matches very well the feature detected in the SDSS data.