Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

Details Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

2002-06-21

arxiv.org/abs/astro-ph/0206390v2

Astrophys.J. 584 (2003) 621-632

Astronomy and Astrophysics

Astrophysics

29 pages, 4 figures. Accepted for publication in Ap.J. (Feb. 20, 2003 issue; v. 584); minor revisions, results unchanged

Scientific paper

10.1086/345738

We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize HI and HeII at redshifts, $z$ of about 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 HeIII reaches a maximum of about 60 percent at $z$ of about 5.6 if Pop III star formation lasts for $10^9$ yr. Future data on HI reionization can test the amount of small-scale power available to the formation of the first objects, and provide a constraint on values of $\sigma_8$ less than or about 0.7. Since current UV observations indicate an epoch of reionization for HeII at $z$ of about 3, HeII may reionize more than once. Measurements of the HeII Gunn-Peterson effect in the intergalactic medium at redshifts exceeding about 3 may reveal the significance of Pop III stars for HeII reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity.

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