Candels: The Contribution of the Observed Galaxy Population to Cosmic Reionization

Abstract

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 WFC3 Early Release Science programs. In contrast to the majority of previous analyses, which assume that the distribution of galaxy ultraviolet (UV) luminosities follows a Schechter distribution, and that the distribution continues to luminosities far below our observable limit, we investigate the contribution to reionization from galaxies which we can observe, free from these assumptions. 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 (f esc ) is ∼30%. A number of previous studies have measured UV luminosity densities at these redshifts that vary by a factor of 5, 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 f esc were present. The observed UV luminosity density from our observed galaxy samples at z = 7 and 8 is not sufficient to maintain a fully reionized IGM unless f esc > 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 f esc 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 f esc < 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 M UV = −13. These escape fractions are sufficient to complete reionization 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 see them, providing an indirect constraint on the global ionizing photon escape fraction.

The size-luminosity relation atz = 7 in CANDELS and its implication on reionization

“…A similar conclusion has been reached also by Finkelstein et al (2012). Since all these conditions are extreme assumptions, we start posing some doubts that the galaxies alone at z ∼ 7 are able to keep the Universe ionized without the additional contributions of faint AGNs (Fiore et al 2011) or other more exotic explanations (Dopita et al 2011;Conroy & Kratter 2012).…”

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…This calculation shows that ultrafaint galaxies can in principle provide enough photons to fully reionize the universe by z ∼ 6 to match observations of the Lyα forest (Fan et al 2006). Both magnitude limits are broadly consistent with a range of constraints from observations, within the reionization model uncertainty: UV luminosity densities (Finkelstein et al 2012) for observable galaxies; quasar near zones (Venemans et al 2015); quasar spectra damping wings (Schroeder et al 2013); GRB spectra damping wings (McQuinn et al 2008); transmission (Fan et al 2006) and dark gaps (McGreer et al 2015) in the Lyα forest; and the clustering of Lyα emitting galaxies (Ouchi et al 2010).…”

Spatially adaptive radiation-hydrodynamical simulations of galaxy formation during cosmological reionization

“…While we compare to observations of dust-corrected SFRs, there are large systematic uncertainties in the amount of dust as well as its properties, especially at the high redshifts of interest here (e.g., Smit et al 2012;Duncan et al 2014). It is also important to keep in mind that observational estimates of the SFR function are subject to cosmic variance (e.g., Finkelstein et al 2012).…”