Population-III ( Pop-III ) starformation ( 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 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 \sim 10 and may have extended to redshifts as low as z \sim 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 , \tau _ { es } \leq 0.05 - 0.06 and a rapid reionization at z \sim 6 are expected with the mean neutral fraction quickly exceeding 50 \% at z \sim 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 \tau _ { es } = 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 , \tau _ { es } = 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 .