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

Steven L. Finkelstein, Anson D'Aloisio, Jan-Pieter Paardekooper, Russell Ryan Jr, Peter Behroozi, Kristian Finlator, Rachael Livermore, Phoebe R. Upton Sanderbeck, Claudio Dalla Vecchia, Sadegh Khochfar

Research output: Contribution to journalArticlepeer-review


 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 <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/M ) ~ 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 subdominant contribution by AGNs 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 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 (~1σ–2σ) tension with observations of Lyα emitters at z ~ 7 and the damping-wing analyses of the two known z > 7 quasars, which prefer  ~ 40%–50%.
Original languageEnglish
Article number36
Number of pages34
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2 Jul 2019


  • astro-ph.CO
  • astro-ph.GA
  • dark ages
  • reionization
  • first stars – early universe – galaxies
  • evolution – galaxies
  • formation


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