Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction
Abstract
We explore scenarios for reionizing the intergalactic medium with low galaxy ionizing photon escape fractions. We combine simulationbased halo massdependent escape fractions with an extrapolation of the observed galaxy restultraviolet luminosity functions to solve for the reionization history from z = 20\to 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 (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 = 45 (and below, when extrapolated), in contrast to some models that assume a single escape fraction. Our predicted ionized volumefilling fraction at z = 7 of {Q}_{{{{H}}}_{II}}} = 78% (±8%) is in modest (∼1σ2σ) tension with observations of Lyα emitters at z ∼ 7 and the dampingwing analyses of the two known z > 7 quasars, which prefer {Q}_{{{{H}}}_{II}},z=7} ∼ 40%50%.
 Publication:

The Astrophysical Journal
 Pub Date:
 July 2019
 DOI:
 10.3847/15384357/ab1ea8
 arXiv:
 arXiv:1902.02792
 Bibcode:
 2019ApJ...879...36F
 Keywords:

 dark ages;
 reionization;
 first stars;
 early universe;
 galaxies: evolution;
 galaxies: formation;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 45 pages, 21 figures, accepted for publication in the Astrophysical Journal