CANDELS: THE CONTRIBUTION OF THE OBSERVED GALAXY POPULATION TO COSMIC REIONIZATION

We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6 ≲ z ≲ 8. These galaxies were selected from new deep near-infrared Hubble Space Telescope imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble UltraDeep Field 2009, and Wide Field Camera 3 Early Release Science programs. We investigate the contribution to reionization from galaxies that we observe directly, thus sidestepping the uncertainties inherent in complementary studies that have invoked assumptions regarding the intrinsic shape or the faint-end cutoff of the galaxy ultraviolet (UV) luminosity function. Due to our larger survey volume, wider wavelength coverage, and updated assumptions about the clumping of gas in the intergalactic medium (IGM), we find that the observable population of galaxies can sustain a fully reionized IGM at z = 6, if the average ionizing photon escape fraction (fesc) is ∼30%. Our result contrasts with a number of previous studies that have measured UV luminosity densities at these redshifts that vary by a factor of five, with many concluding that galaxies could not complete reionization by z = 6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of fesc were present. The specific UV luminosity density from our observed galaxy samples at z = 7 and 8 is not sufficient to maintain a fully reionized IGM unless fesc > 50%. We examine the contribution from galaxies in different luminosity ranges and find that the sub-L* galaxies we detect are stronger contributors to the ionizing photon budget than the L > L* population, unless fesc is luminosity dependent. Combining our observations with constraints on the emission rate of ionizing photons from Lyα forest observations at z = 6, we find that we can constrain fesc < 34% (2σ) if the observed galaxies are the only contributors to reionization, or <13% (2σ) if the luminosity function extends to a limiting magnitude of MUV = −13. These escape fractions are sufficient to sustain an ionized IGM by z = 6. Current constraints on the high-redshift galaxy population imply that the volume ionized fraction of the IGM, while consistent with unity at z ⩽ 6, appears to drop at redshifts not much higher than 7, consistent with a number of complementary reionization probes. If faint galaxies dominated the ionizing photon budget at z = 6–7, future extremely deep observations with the James Webb Space Telescope will probe deep enough to directly observe them, providing an indirect constraint on the global ionizing photon escape fraction.