First Light in the Universe: Primordial Stars and Reionization

The universe recombined at a redshift z~1000, about 400,000 years after the big bang, changing its composition from an ionized plasma to a neutral gas. We can see the universe as it was at this time through the Cosmic Microwave Background (CMB), as recently revealed in unprecedented detail by the Wilkinson Microwave Anisotropy Probe (WMAP). Although the universe was rather uniform at recombination, gravitational forces caused slightly denser regions of the universe to collapse under their own weight, eventually forming large clumps, known as minihalos. It is thought that the centers of some of these minihalos were the sites of the first star formation. The light from these first stars began to re-ionize the universe when neutral hydrogen atoms absorbed the light and their electrons became unbound from their protons. Analysis of data obtained by WMAP indicates that reionization must have made substantial progress by redshifts z > 17, less than ~200 million years after the big bang. This implies that the first few generations of stars may have contributed to reionization much more than previously thought. How and when reionization occurred is one of the great puzzles confronting astronomers and cosmologists today.

In order to understand the radiative feedback effects from the first stars in detail, we have undertaken three-dimensional simulations of the propagation of ionization fronts around massive Population III stars (stellar masses about 100-200 solar masses) that form at a redshift z=20. We follow the evolution of ionized regions created by these stars within the density distribution resulting from a smoothed particle hydrodynamics (SPH) simulation of primordial star formation. We describe the structure and evolution of the ionized regions and discuss implications for reionization and subsequent star formation.

Abstract Author(s): Marcelo Alvarez<br />Volker Bromm<br />Paul Shapiro