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

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

Research output: Contribution to journalArticlepeer-review

182 Scopus citations


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 o) ∼ 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 (US)
Article number36
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 1 2019


  • Dark ages
  • early universe
  • first stars
  • galaxies: evolution
  • galaxies: formation
  • reionization

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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