History of Hydrogen Reionization in the Cold Dark Matter Model

We calculate the reionization history in Cold Dark Matter (CDM) models. Theepoch of the end of reionization and the Thomson scattering optical depth tothe cosmic microwave background depend on the power spectrum amplitude on smallscales and on the ionizing photon emissivity per unit mass in collapsed halos.We calibrate the emissivity to reproduce the measured ionizing backgroundintensity at z=4. Models in which all CDM halos have either a constantemissivity or a constant energy emitted per Hubble time, per unit mass, predictthat reionization ends near z~6 and the optical depth is in the range 0.05 15 must have ionizing emissivities per unit masslarger by a factor >~ 50 compared to the more massive halos that produce theionizing emissivity at z=4. This factor increases to 100 if the CDM powerspectrum amplitude is required to agree with the Croft et al. (2002)measurement from the Lyman alpha forest. If tau_e >~ 0.17 were confirmed, ahigher ionizing emissivity at z>15 compared to z=4 might arise from an enhancedstar formation rate or quasar abundance per unit mass and an increased escapefraction for ionizing photons; the end of reionization could have been delayedto z~6 because of the suppression of gas accretion and star formation inlow-mass halos as the medium was reionized.Comment: 19 pages, 4 figues, submitted to Ap