Detecting the Rise and Fall of the First Stars by Their Impact on Cosmic Reionization

The intergalactic medium was reionized before redshift z ~ 6, most likely by starlight which escaped from early galaxies. The very ?rst stars formed when hydrogen molecules (H2) cooled gas inside the smallest galaxies, minihalos of mass between 105 and 108 solar masses, before redshift z ~ 40. Minihalo stars then started reionization but could not ?nish it before the rising background of H2-dissociating soft-ultraviolet starlight choked them o?. We con?rm this hypothesis by the ?rst large-scale radiative transfer simulations to include minihalo sources and their suppression. We show that reionization began much earlier with minihalo sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization ?uctuations of the cosmic microwave background signi?cantly. Although within current WMAP uncertainties, this boost should be readily detectable by Planck. If reionization ended as late as z ov 7, as suggested by other observations, Planck will thereby see the signature of the ?rst stars at high redshift, currently undetectable by any other probe. We also show that minimal reionization models satisfying both the late reionization condition, zov . 7, and the large optical depth condition, tes & 0.085, can be distinguished from our ?ducial model under the same constraints by Planck at high con?dence level.