Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

We examine the significance of the first metal-free stars (Pop 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 Pop III stellar cluster for H I and He II being 1.6 and 10 times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize H I and He II at redshifts, z ≃ 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Pop III and Pop 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 Pop III star formation lasts for 10 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 Pop III stars for He II reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity. Subject headings: cosmology: theory—intergalactic medium NSF Astronomy and Astrophysics Postdoctoral Fellow Present address: Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637 Also at JILA, University of Colorado and National Institute of Standards and Technology