The era of massive Population III stars: Cosmological implications and self-termination

The birth and death of the first generation of stars have important implications for the thermal state and chemical properties of the intergalactic medium (IGM) in the early universe. Sometime after recombination, the neutral chemically pristine gas was reionized by ultraviolet photons emitted from the first stars but was also enriched with heavy elements when these stars ended their lives as energetic supernovae. Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal polluted, as well as ionized, by massive Population III stars formed in small-mass (similar to10(6)-10(7) M-circle dot) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to similar to10(53) ergs, the volume-averaged mean metallicity will quickly reach Z similar to 10(-4) Z(circle dot) by a redshift of similar to15-20, possibly causing a transition to the formation of a stellar population that is dominated by low-mass stars. In this scenario, the early chemical enrichment history should closely trace the reionization history of the IGM, and the end of the Population III era is marked by the completion of reionization and preenrichment by z similar to 15. We conclude that, while the preenrichment may partially account for the "metallicity floor" in high-redshift Lyalpha clouds, it does not significantly affect the elemental abundance in the intracluster medium.