EFFECT OF HALO BIAS AND LYMAN LIMIT SYSTEMS ON THE HISTORY OF COSMIC REIONIZATION

We extend the existing analytical model of reionization by Furlanetto et al. to include the biasing of reionization sources and additional absorption by Lyman limit systems. Both effects enhance the original model in non-trivial ways, but do not change its qualitative features. Our model is, by construction, consistent with the observed evolution of the galaxy luminosity function at z less than or similar to 8 and with the observed evolution of Ly alpha forest at z less than or similar to 6. We find that the same model can match the Wilkinson Microwave Anisotropy Probe/Planck constraint on the Thompson optical depth and the South Pole Telescope and EDGES constraints on the duration of reionization for values of the relative escape fraction that are consistent with the observational measurements at lower redshifts. However, such a match is only possible if dwarf galaxies contribute substantially to the ionizing photon budget. The latter condition is inconsistent with simulations and observational upper limits on the escape fraction from dwarfs at z similar to 3. Whether such a disagreement is due to the different nature of z > 6 galaxies, the inadequacy of simulations and/or some of the observational constraints, or indicates an additional source of ionizing radiation at z > 8 remains to be seen.