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We explore scenarios for reionizing the intergalactic medium with low galaxyionizing photon escape fractions. We combine simulation-based halo-massdependent escape fractions with an extrapolation of the observed galaxyrest-ultraviolet luminosity functions to solve for the reionization historyfrom z=20 to z=4. We explore the posterior distributions for key unknownquantities, including the limiting halo mass for star-formation, the ionizingphoton production efficiency, and a potential contribution from active galacticnuclei (AGN). We marginalize over the allowable parameter space using a MarkovChain Monte Carlo method, finding a solution which satisfies the mostmodel-independent constraints on reionization. Our fiducial model can matchobservational constraints with an average escape fraction of <5% throughout thebulk of the epoch of reionization if: i) galaxies form stars down to the atomiccooling limit before reionization and a photosuppression mass oflog(M_h/Msol)~9 during/after reionization (-13<M_UV,lim<-11); ii) galaxiesbecome more efficient producers of ionizing photons at higher redshifts andfainter magnitudes, and iii) there is a significant, but sub-dominant,contribution by AGN at z < 7. In this model the faintest galaxies (M_UV>-15)dominate the ionizing emissivity, leading to an earlier start to reionizationand a smoother evolution of the ionized volume filling fraction than modelswhich assume a single escape fraction at all redshifts and luminosities. Theionizing emissivity from this model is consistent with observations at z=4-5(and below, when extrapolated), in contrast to some models which assume asingle escape fraction. Our predicted ionized volume filling fraction at z=7 ofQ_HII=78% (+\- 8%) is in ~1-2 sigma tension with observations of Lya emittersat z~7 and the damping wing analyses of the two known z>7 quasars, which preferQ_HII,z=7~40-50%.

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