Detection of He II reionization in the SDSS quasar sample
  The expansion of the Universe leads to a rapid drop in the mean hydrogen Ly-alpha opacity tau of the intergalactic medium (IGM), tau (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 temperature increase associated with He II reionization leads to a relatively sudden decrease in tau around the reionization epoch of ~ 10 per cent. We find clear evidence for such a feature in the evolution of tau determined from a sample of ~ 1100 quasars obtained from the SDSS. He II reionization starts at redshift ~ 3.4, and lasts for (Delta z) ~ 0.4. The increase in the IGM temperature also explains the widths of hydrogen absorption lines as measured in high-resolution spectra. 
T. Theuns, M. Bernardi, J. Frieman, P. Hewett, J. Schaye, R. K. Sheth,   and M. Subbarao