We examine the significance of the first metal-free stars (Population III) for the cosmological reionization of H I and He II. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Population III stellar cluster for H I and He II being 1.6 and 105 times stronger, respectively, than those from a Population II cluster. For the currently favored cosmology, we find that Population III stars alone can reionize H I at redshifts of z 9 and 4.7 and He II at z 5.1 and 0.7 for continuous and instantaneous modes of star formation, respectively. More realistic scenarios involving combinations of Population III and Population II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of He III reaches a maximum of about 60% at z ~ 5.6 if Population III star formation lasts for 109 yr. Future data on H I reionization can test the amount of small-scale power available to the formation of the first objects and provide a constraint on values of σ8 0.7. Since current UV observations indicate an epoch of reionization for He II at z ~ 3, He II may reionize more than once. Measurements of the He II Gunn-Peterson effect in the intergalactic medium at redshifts z 3 may reveal the significance of Population III stars for He II reionization, particularly in "void" regions that may contain "relic" ionization from early Population III stellar activity.
Aparna Venkatesan, Jason Tumlinson and J. Michael Shull. Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization. The Astrophysical Journal, 584:621-632, 2003 February 20. http://dx.doi.org/10.1086/345738