Three Epochs of Star Formation in the High‐Redshift Universe

We investigate the impact of an early population of massive stars on theirsurroundings. Dissociation of molecular hydrogen by strong UV emission fromsuch stars is expected to produce a global transition in the cooling mechanismof minihalos at a redshift of approximately 30, strongly inhibiting starformation until more massive halos can collapse. Furthermore, chemicalenrichment from Pop III supernovae produces a second transition at z ~ 15-20,when the mean metallicity of the universe exceeds a critical threshold and PopIII star formation gives way to Pop II. We show that observations of highredshift supernovae with the Next Generation Space Telescope (NGST) have thepotential to trace the cosmic star formation rate out to z >~ 20, provided thatPop III supernovae are at least as bright as, and ideally brighter than, typeIa supernovae. We also propose a mechanism for the formation of a novelpopulation of extremely low metallicity stars of intermediate mass at very highredshifts, which we term Pop II.5. In our model shock compression, heating, andsubsequent cooling to high density reduces the fragment mass in primordial gasto ~10 M_sun, allowing low mass stars to form. We predict the number density ofrelic Pop II.5 stars in the Milky Way halo today and find that, with certainassumptions, there should be ~ 10 kpc^{-3} in the solar neighborhood.Comment: 31 pages, 6 figures, revised and enlarged version, ApJ in press (March 20, 2003