Experimental Constraints on Reionization Models

A self-consistent formalism to jointly study cosmic reionization and thermal history of the intergalactic medium (IGM) in a λCDM cosmology is presented. The model implements most of the relevant physics governing these processes, such as the inhomogeneous IGM density distribution, three different classes of ionizing photon sources [massive Population III (PopIII) stars, Population II (PopII) stars and quasi-stellar objects (QSOs)], and radiative feedback inhibiting star formation in low-mass galaxies. By constraining the model free parameters with available data on redshift evolution of Lyman-limit absorption systems, Gunn-Peterson and electron scattering optical depths, near-infrared background and cosmic star formation history, we select a fiducial model, whose main predictions are as follows. Hydrogen was completely reionized at z≈ 15, while He ii must have been reionized by z≈ 12, allowing for the uncertainties in the ionizing photon efficiencies of stars. At z≈ 7, He iii suffered an almost complete recombination as a result of the extinction of PopIII stars, as required by the interpretation of the NIRB. A QSO-induced complete He ii reionization occurs at z= 3.5; a similar double H reionization does not take place due to the large number of photons with energies >13.6 eV from PopII stars and QSOs, even after all PopIII stars have disappeared. Following reionization, the temperature of the IGM corresponding to the mean gas density, T0, is boosted to 1.5 × 104 K; following that it decreases with a relatively flat trend. Observations of T0 are consistent with the fact that He is singly ionized at z≳ 3.5, while they are consistent with He being doubly ionized at z≲ 3.5. This might be interpreted as a signature of (second) He ii reionization. Only 0.3 per cent of the stars produced by z= 2 need to be PopIII stars in order to achieve the first hydrogen reionization. In addition, we get useful constraints on the ionizing photon efficiencies (which are a combination of the star-forming efficiency and the escape fraction of ionizing photons from collapsed haloes) of PopII and PopIII stars, namely, εPopII < 0.01, 0.002 < εPopIII < 0.03. Varying the efficiencies in these two ranges does not affect the scenario described above. Such a model not only relieves the tension between the Gunn-Peterson optical depth and WMAP observations, but also accounts self-consistently for all known observational constraints. We discuss how the results compare with recent numerical reionization studies and other theoretical arguments.