The extended star formation history of the first generation of stars and the reionization of cosmic hydrogen

Population III star formation (SF) is thought to be quenched when the metallicity of the star-forming gas has reached a critical level. At high redshift, when the general intergalactic medium (IGM) was enriched with metals, the fraction of primordial gas that had already collapsed in minihalos was significantly larger than the fraction of primordial gas that had already been involved in Population III SF. We argue that this reservoir of minihalo gas remained largely in a metalfree state until these minihalos merged into large systems and formed stars. As a result, the era of Population III SF was significantly prolonged, leading to a total integrated Population III SF an order of magnitude larger than expected for an abrupt transition redshift. We find that the contribution of Population 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 photoionization, and screening of reionization by minihalos. Nevertheless, the extended epoch of Population III SF may result in complex reionization histories. The relative contribution of Population III stars to reionization can be quantified and will be tested by three-year WMAP results, showing (1) if Population III stars do not contribute to reionization, τ ≤ 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 Population III stars is significantly larger than that for Population II stars, then a maximum τ_cs = 0.21 is achievable; and (3) where the product of star formation efficiency and escape fraction for Population III stars is comparable to that for Population II stars, τ_s = 0.09-0.12, 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.