Hierarchical Growth and Cosmic Star Formation: Enrichment, Outflows, and Supernova Rates

Abstract

The cosmic star formation histories are evaluated for different minimum masses of the initial halo structures, with allowance for realistic gas outflows. With a minimum halo mass of 107-108 Msolar and a moderate outflow efficiency, we reproduce both the current baryon fraction and the early chemical enrichment of the IGM. The intensity of the formation rate of ``normal'' stars is also well constrained by the observations: it has to be dominated by star formation in elliptical galaxies, except perhaps at very low redshift. The fraction of baryons in stars is predicted as are also the Type Ia and II supernova event rates. Comparison with SN observations in the redshift range z=0-2 allows us to set strong constraints on the time delay of Type Ia supernovae (a delay of ~3 Gyr is required to fit the data), the lower end of the mass range of the progenitors (2-8 Msolar), and the fraction of white dwarfs that reproduce the Type Ia supernova (about 1%). The intensity of zero metallicity star formation below 270 Msolar must be limited in order to avoid premature overenrichment of the IGM. About 50% of the metals present in the IGM at z=0 have been produced by Population III stars at very high z. The remaining 50% are ejected later by galaxies forming normal stars. We conclude that about 10-3 of the mass in baryons must lie in the first massive stars in order to produce enough ionizing photons to allow early reionization of the IGM by z~15.