REIONIZATION DURING HIERARCHICAL-CLUSTERING IN A UNIVERSE DOMINATED BY COLD DARK-MATTER

We investigate reionization of the Universe by early formation of stars and quasars in the hierarchical clustering scheme of the cold dark matter (CDM) scenario, with perturbation fluctuations originating from a Harrison-Zeldovich-type primordial spectrum normalized by the COBE data. It is found that ionizing UV flux from OB stars, with the abundance given by the standard initial mass function, is strong enough to ionize the Universe from redshift z almost-equal-to 30 to the present epoch, if 1-2 per cent of the collapsed baryons go into stars. This lessens significantly the cosmic microwave background radiation (CMB) anisotropies at small angular scales. Reionization also increases the Jeans mass to M(luminous) almost-equal-to 10(9) M. for z less than or similar to 10, which leads to a cut-off in the luminosity function of normal galaxies on the faint side. A strong UV flux is expected at z almost-equal-to 2-5, and the null result of the Gunn-Peterson test is naturally explained. Early star formation also results in the production of heavy elements, and the observed metal abundance places a strong constraint on the photon energy injection into intergalactic space.