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

Abstract 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 = . 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 = 78% (±8%) is in modest (∼1 σ –2 σ ) tension with observations of Ly α emitters at z ∼ 7 and the damping-wing analyses of the two known z > 7 quasars, which prefer ∼ 40%–50%.