Theoretical studies and current observations of the high-redshift intergalactic medium (IGM) indicate that at least two cosmic transitions occur by the time the universe reaches gas metallicities of about 10-3 Z☉. One is the cosmological reionization of the IGM, and the second is the transition from a primordial to present-day mode of star formation. We quantify this relation through new calculations of the ionizing radiation produced in association with the elements carbon, oxygen, and silicon observed in Galactic metal-poor halo stars, which are likely second-generation objects formed in the wake of primordial supernovae. We demonstrate that sufficient ionizing photons per baryon are created by enrichment levels of [Fe/H] ~ -3 in the environment of metal-poor halo stars that provide the optical depth in the cosmic microwave background of ~0.1 detected by WMAP. We show that, on a star-by-star basis, a genuine cosmic milestone in the ionization of the IGM and in the mode of star formation occurred at metallicities of 10-4 to 10-3 Z☉ in these halo stars. This provides us with an important link in the chain of evidence for metal-free first stars having dominated the process of reionization by z ~ 6. We conclude that many of the Fe-poor halo stars formed close to the end of or soon after cosmological reionization, making them the ideal probe of the physical conditions under which the transition from first- to second-generation star formation happened in primordial galaxies.
Aparna Venkatesan. A cosmic milestone: Constraints from metal-poor halo stars on the cosmological reionization epoch. 2006 Astrophysical Journal. 641 L81 doi:10.1086/504037