TY - JOUR

T1 - Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction

AU - Finkelstein, Steven L.

AU - D'Aloisio, Anson

AU - Paardekooper, Jan Pieter

AU - Ryan, Russell

AU - Behroozi, Peter

AU - Finlator, Kristian

AU - Livermore, Rachael

AU - Sanderbeck, Phoebe R.Upton

AU - Vecchia, Claudio Dalla

AU - Khochfar, Sadegh

N1 - Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved.

PY - 2019/7/1

Y1 - 2019/7/1

N2 - 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%.

AB - 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%.

KW - Dark ages

KW - early universe

KW - first stars

KW - galaxies: evolution

KW - galaxies: formation

KW - reionization

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U2 - 10.3847/1538-4357/ab1ea8

DO - 10.3847/1538-4357/ab1ea8

M3 - Article

AN - SCOPUS:85069478415

VL - 879

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1

M1 - 36

ER -