Cosmological Reionization Caused by Structure Formation

Abstract

Various observations suggest that the universe is reionized at z 10 is possible if the power spectrum of the density fluctuations is sufficiently steep (Sasaki, Takahara and Suto, 1993). In this paper, we performed detailed numerical calculations based on the scenario and confirmed the results of the previous paper. In hierarchical clustering models of structure formation, numerous nonlinear objects on small scales would collapse much earlier than a redshift z~ 10. Our scenario assumes that such objects dissipate their kinetic energy efficiently into heat and eventually provide copious ionizing photons which reionize the universe. We assume that the power spectrum of density fluctuation· obeys a single power-law P(k)cxkn on scales of interest at the standard recombination epoch. We find that in models with the spectral index n= -1 as long as the velocity dispersion at the present epoch is moderately large, the universe can be reionized at z='5-20. For flatter spectra or lower efficiency, the universe remains effectively neutraL We also obtain the spectrum of background radiation at the present epoch. Furthermore, we compare our numerical results with various observations such as the Gunn·Peterson test, UV flux at z~3 and the present intensity of background radiation at the optical, UV and soft X-ray bands.