Abstract
We explore scenarios for reionizing the intergalactic medium with low
galaxy ionizing photon escape fractions. We combine simulationbased
halo mass–dependent escape fractions with an extrapolation of the
observed galaxy restultraviolet 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 modelindependent 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 volumefilling
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 volumefilling fraction at z = 7 of = 78% (±8%) is in modest (~1σ–2σ) tension with observations of Lyα emitters at z ~ 7 and the dampingwing analyses of the two known z > 7 quasars, which prefer ~ 40%–50%.
Original language  English 

Article number  36 
Number of pages  34 
Journal  Astrophysical Journal 
Volume  879 
Issue number  1 
DOIs  
Publication status  Published  2 Jul 2019 
Keywords
 astroph.CO
 astroph.GA
 dark ages
 reionization
 first stars – early universe – galaxies
 evolution – galaxies
 formation
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Sadegh Khochfar
 School of Physics and Astronomy  Personal Chair of Theoretical Astrophysics
Person: Academic: Research Active