THE REIONIZATION HISTORY IN THE LOGNORMAL MODEL

摘要

We study the evolution of baryonic gas before reionization in the lognormal (LN) model of cosmic clustering. We show that the thermal history of the universe around reionization can roughly be divided into three epochs: (1) a cold dark age z > z_r, in which the baryon gas is neutral and opaque to Lyα photons; (2) a hot dark age z_r > z > Z_(GP), in which a predominant part of the baryon gas is ionized and hot but is still opaque to Lyα photons; and (3) a bright age z < Z_(GP), in which the universe is ionized enough to be transparent to Lyα photons. In the flat cold dark matter cosmological models given by WMAP and COBE, the difference of the two redshifts z_r - Z_(GP) is found to be as large as ～10, with z_r ～ 17 and z_(Gp) ～ 7. This reionization history naturally yields a high optical depth of the CMB τ_e approx= 0.12-0.19, which is observed in the TE polarization in the WMAP data, and a low redshift Z_(GP) for the appearance of the Lyα Gunn-Peterson trough, z_(Gp) approx= 6-8, in QSO absorption spectra. The universe stays so long in an ionized, yet Lyα opaque, stage because the first photoionization heats the inter-galactic gas effectively and balances the gravitational clustering for a long period of time. Therefore, a high τ_e and low z_(Gp) is a result of all the considered models. Besides the cosmological parameters, the only free parameter we used in the calculation is N_(ion), the mean number of ionization photons produced by each baryon in collapsed objects. We take it to be 40-80 in the calculations.