The Ionizing Efficiency of the First Stars

We investigate whether a single population of first stars could have influenced both the metal enrichment and reionization of the high-redshift intergalactic medium (IGM) by calculating the generated ionizing radiation per unit metal yield as a function of the metallicity of stellar populations. We examine the relation between the ionizing radiation and carbon created by the first stars, since the evidence for the widespread enrichment of the IGM at redshifts z ~ 3-4 comes from the detection of C IV absorption. We find that the number of ionizing photons per baryon generated in association with the detected IGM metallicity may well exceed that required for a late hydrogen reionization at z ~ 6, by up to a factor of 10-20 for metal-free stars in a present-day initial mass function (IMF). This would be in agreement with similar indications from recent observations of the microwave background and the high-z IGM. In addition, the contribution from intermediate-mass stars to the total metal yield, neglected in past works, substantially impacts such calculations. Finally, a top-heavy IMF is not necessarily preferred as a more efficient high-z source of ionizing radiation, based on nucleosynthetic arguments in association with a given level of IGM enrichment.