Early reionization by the first galaxies

Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, τe∼ 0.17. The reionization of the Universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results (Ωm= 0.3, ΩΛ= 0.7, h= 0.7, Ωb= 0.04, n= 1 and σ8= 0.9). Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with ionizing photon production as expected for zero-metallicity stars distributed according to a standard stellar initial mass function (IMF) (1061 photons per unit solar mass of formed stars) and with a moderate photon escape fraction from galaxies (5 per cent), produces τe= 0.104, which is within 1.0 to 1.5σ of the ‘best’WMAP value. Values of up to 0.16 can be produced by taking larger escape fractions or a top-heavy IMF. The data do not require a separate populations of ‘miniquasars’ or of stars forming in objects with total masses below 109 M⊙. Reconciling such early reionization with the observed Gunn–Peterson troughs in z > 6 quasars may be challenging. Possible resolutions of this problem are discussed.