Signatures of stellar reionization of the universe

The high ionization level and nonzero metallicity (similar to 1% Z(.)) of the intergalactic gas at redshifts z less than or similar to 5 implies that nonlinear structure had started to form in the universe at earlier times than we currently probe. In cold dark matter (CDM) cosmologies, the first generation of baryonic objects emerges at redshifts z similar to 10-50. Here we examine the observable consequences of the possibility that an early generation of stars reionized the universe and resulted in the observed metallicity of the Ly alpha forest. Forthcoming microwave anisotropy experiments will be sensitive to the damping of anisotropies caused by scattering off free electrons from the reionization epoch. For a large range of CDM models with a Scalo stellar mass function, we find that reionization occurs at a redshift z greater than or similar to 10 and damps the amplitude of anisotropies on angular scales less than or similar to 10 degrees by a detectable amount, similar to 10%-25%. However, reionization is substantially delayed if the initial stellar mass function transformed most of the baryons Into low-mass stars. In this case, the mass fraction of pregalactic stars could be constrained from the statistics of microlensing events in galactic halos or along lines of sight to quasars. Deep infrared imaging with future space telescopes (such as the Space Infrared Telescope Facility or the Next Generation Space Telescope) will be able to detect bright star clusters at z greater than or similar to 5. The cumulative bremsstrahlung emission from these star clusters yields a measurable distortion to the spectrum of the microwave background.