The Extended Starformation History of the First Generation of Stars, and the Reionization of Cosmic Hydrogen

Population-III (Pop-III) starformation (SF) is thought to be quenched when the metallicity of the starforming gas has reached a critical level. At high redshift, when the general intergalactic medium (IGM) was enriched with metals, the fraction of primordial gas already collapsed in minihalos (above the Jeans mass but below the mass corresponding to the efficient atomic cooling threshold for SF) was significantly larger than the fraction of primordial gas that had already been involved in Pop-III SF. We argue that this reservoir of minihalo gas remained largely in a metal-free state, until these minihalos merged into large systems (above the hydrogen cooling threshold) and formed stars. As a result, the era of Pop-III SF was significantly prolonged, leading to a total integrated Pop-III SF that was an order of magnitude larger than expected for an abrupt transition redshift. We find that the contribution of Pop-III SF to the reionization of hydrogen could have been significant until z ∼ 10 and may have extended to redshifts as low as z ∼ 6. Our modeling allows for gradual enrichment of the IGM, feedback from photo-ionization and screening of reionization by minihalos. Nevertheless, the extended epoch of Pop-III SF may result in complex reionization histories, where multi peaks are possible over some regions of parameter space. The relative contribution of Pop-III stars to reionization can be quantified and will be tested by three-year WMAP results: (1) if Pop-III stars do not contribute to reionization, τes ≤ 0.05 − 0.06 and a rapid reionization at z ∼ 6 are expected with the mean neutral fraction quickly exceeding 50% at z ∼ 8; (2) if the product of star formation efficiency and escape fraction for Pop-III stars is significantly larger than that for Pop-II stars, then a maximum τes = 0.21 is achievable; (3) in a perhaps more plausible scenario where the product of star formation efficiency and escape fraction for Pop-III stars is comparable to that for Pop-II stars, τes = 0.09 − 0.12 would be observed, with reionization histories characterized by an extended ionization plateau from z = 7− 12 where the mean neutral fraction stays in a narrow range of 0.1− 0.3. This result holds regardless of the redshift where the IGM becomes enriched with metals. Subject headings: Cosmology: theory – Pop-III stars – Early universe – Intergalactic Medium