Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction

We explore scenarios for reionizing the intergalactic medium with low galaxy ionizing photon escape fractions. We combine simulation-based halo mass-dependent escape fractions with an extrapolation of the observed galaxy rest-ultraviolet luminosity functions to solve for the reionization history from z = 20 -> 4. We explore the posterior distributions for key unknown quantities, including the limiting halo mass for star formation, the ionizing photon production efficiency, and a potential contribution from active galactic nuclei (AGNs). We marginalize over the allowable parameter space using a Markov chain Monte Carlo method, finding a solution that satisfies the most model-independent constraints on reionization. Our fiducial model can match observational constraints with an average escape fraction of -15) dominate the ionizing emissivity, leading to an earlier start to reionization and a smoother evolution of the ionized volume-filling fraction than models that assume a single escape fraction at all redshifts and luminosities. The ionizing emissivity from this model is consistent with observations at z = 4-5 (and below, when extrapolated), in contrast to some models that assume a single escape fraction. Our predicted ionized volume-filling fraction at z = 7 of Q(HII) = 78% (+/- 8%) is in modest (similar to 1 sigma-2 sigma) tension with observations of Ly alpha emitters at z similar to 7 and the damping-wing analyses of the two known z > 7 quasars, which prefer Q(HII, z) = 7 similar to 40%-50%.National Science Foundation through AAG award [AST 1518183]; HST award [HST-AR-15013.005-A]; European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) via the ERC Advanced Grant "STARLIGHT: Formation of the First Stars" [339177]; Spanish Ministry of Economy and Competitiveness (MINECO) [RYC-2015-1807]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu