A SIMPLE-MODEL OF COSMOLOGICAL REIONIZATION

A simple scenario of cosmological reionization is presented. In hierarchical clustering models of structure formation, numerous nonlinear objects on small scales would collapse much earlier than a redshift z approximately 10. Our scenario assumes that such objects dissipate their kinetic energy efficiently into heat and eventually provide copious ionizing photons which reionize the universe almost instantaneously. We derive a simple estimate of the energy conversion efficiency required for the full reionization. Then we apply the formula to a specific hierarchical clustering model in which the power spectrum of density fluctuation obeys a single power-law P(k) is-proportional-to k(n) approximately on scales of interest (at standard recombination epoch). After comparing the resulting production rate of ionizing photons with cooling rates of the relevant processes, we find that models with the spectral index n greater-than-or-equal-to -1 can potentially reionize the universe at z greater-than-or-equal-to 10. For flatter spectra, the universe remains effectively neutral. This index is somewhat larger than the conventional value around -2 on galactic scales in a cold dark matter universe, but is preferred phenomenologically in primordial isocurvature baryon models, for instance. Therefore it is possible to achieve a reionization of the universe naturally due to the energy release associated with the structure formation. We discuss briefly the implications of our scenario, in particular on the anisotropies and distortion of the cosmic microwave background, and on a characteristic mass of galaxies.