Three Epochs of Star Formation in the High-Redshift Universe

We investigate the impact of an early population of massive stars on their surroundings. Dissociation of molecular hydrogen by strong UV emission from such stars is expected to produce a global transition in the cooling mechanism of minihalos at a redshift of approximately 30, strongly inhibiting star formation until more massive halos can collapse. Furthermore, chemical enrichment from Population III supernovae produces a second transition at z~15-20, when the mean metallicity of the universe exceeds a critical threshold and Population III star formation gives way to Population II. We show that observations of high-redshift supernovae with the Next Generation Space Telescope have the potential to trace the cosmic star formation rate out to z>~20, provided that Population III supernovae are at least as bright as, and ideally brighter than, Type Ia supernovae. We also propose a mechanism for the formation of a novel population of extremely low metallicity stars of intermediate mass at very high redshifts, which we term Population II.5. In our model shock compression, heating, and subsequent cooling to high density reduces the fragment mass in primordial gas to ~10 M_solar, allowing low-mass stars to form. We predict the number density of relic Population II.5 stars in the Milky Way halo today and find that, with certain assumptions, there should be ~10 kpc^-3 in the solar neighborhood.