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 to z=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 (AGN). We marginalize over the allowable parameter space using a Markov
Chain Monte Carlo method, finding a solution which satisfies the most
model-independent constraints on reionization. Our fiducial model can match
observational constraints with an average escape fraction of <5% throughout the
bulk of the epoch of reionization if: i) galaxies form stars down to the atomic
cooling limit before reionization and a photosuppression mass of
log(M_h/Msol)~9 during/after reionization (-13<M_UV,lim<-11); ii) galaxies
become more efficient producers of ionizing photons at higher redshifts and
fainter 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 reionization
and a smoother evolution of the ionized volume filling fraction than models
which 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 which assume a
single escape fraction. Our predicted ionized volume filling fraction at z=7 of
Q_HII=78% (+\- 8%) is in ~1-2 sigma tension with observations of Lya emitters
at z~7 and the damping wing analyses of the two known z>7 quasars, which prefer
Q_HII,z=7~40-50%.
