A Cosmic Milestone: Constraints from Metal-poor Halo Stars on the Cosmological Reionization Epoch

Theoretical studies and current observations of the high-redshiftintergalactic medium (IGM) indicate that at least two cosmic transitions occurby the time the universe reaches gas metallicities of about $10^{-3}$ of solarvalues. These are the cosmological reionization of the IGM, and the transitionfrom a primordial to present-day mode of star formation. We quantify thisrelation through new calculations of the ionizing radiation produced inassociation with the elements carbon, oxygen and silicon observed in Galacticmetal-poor halo stars, which are likely second-generation objects formed in thewake of primordial supernovae. We demonstrate that sufficient ionizing photonsper baryon are created by enrichment levels of [Fe/H] of about -3 in theenvironment of metal-poor halo stars to provide the optical depth in the cosmicmicrowave background of about 0.1 detected by $WMAP$. We show, on a star bystar basis, that a genuine cosmic milestone in IGM ionization and starformation mode occurred at metallicities of $10^{-4}$ to $10^{-3}$ solar inthese halo stars. This provides an important link in the chain of evidence formetal-free first stars having dominated the process of reionization by redshift6. We conclude that many of the Fe-poor halo stars formed close to the end ofor soon after cosmological reionization, making them the ideal probe of thephysical conditions under which the transition from first- to second-generationstar formation happened in primordial galaxies.Comment: ApJ Letters, in press. Minor revisions to reflect new 3-year WMAP data, results unchange