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 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 (∼ 10−10M⊙) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to ∼ 10 ergs, the volume-averaged mean metallicity will quickly reach Z ∼ 10Z⊙ by a redshift of ∼ 15 − 20, possibly causing a prompt 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 pre-enrichment by z ∼ 15. We conclude that, while the pre-enrichment may partially account for the “metallicity-floor” in high-redshift Lyman-α clouds, it does not significantly affect the elemental abundance in the intracluster medium. Subject headings: cosmology: theory — galaxies: formation — intergalactic medium — stars: formation