Reheating during Hierarchical Clustering in the Universe Dominated by the Cold Dark Matter

We investigate reheating of the universe by early formation of stars and quasars in the hierarchical clustering scheme of cold dark matter scenario, with perturbation fluctuations 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 $z\approx 30$ to the present epoch, if 1--2\% of the collapsed baryons go into stars. This lessens significantly the CMB anisotropies at a small angular scale. Reionization also increases the Jeans mass to $M_{\rm luminous}\approx 10^9M_\odot$ for $z\lsim 10$, which leads to a cut-off of the luminosity function of normal galaxies on a faint side. A strong uv flux is expected at $z\approx 2-5$, and the null result of the Gunn-Peterson test is naturally explained. Early star formation also results in production of heavy elements, and the observational metal abundance sets a strong constraint on the photon energy injection into the intergalactic space.