Early reionization by the first galaxies

Abstract

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) (106: 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 10 9 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.