THE LYMAN-CONTINUUM PHOTON PRODUCTION EFFICIENCY ξION OF z ∼ 4–5 GALAXIES FROM IRAC-BASED Hα MEASUREMENTS: IMPLICATIONS FOR THE ESCAPE FRACTION AND COSMIC REIONIZATION

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Abstract
Galaxies represent one of the preferred candidate sources to drive the reionization of the universe. Even as gains are made in mapping the galaxy UV luminosity density to , significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency and the escape fraction f esc. As we show here, the first of these unknowns is directly measurable in z = 4–5 galaxies based on the impact the Hα line has on the observed IRAC fluxes. By computing a LyC photon production rate from the implied Hα luminosities for a broad selection of z = 4–5 galaxies and comparing this against the dust-corrected UV-continuum luminosities, we provide the first-ever direct estimates of the LyC photon production efficiency for the galaxy population. We find to have a mean value of and for sub-L* z = 4–5 galaxies adopting Calzetti and SMC dust laws, respectively. Reassuringly, both derived values are consistent with the standard assumed 's in reionization models, with a slight preference for higher 's (by ∼0.1 dex) adopting the SMC dust law. High values of (∼25.5–25.8 dex) are derived for the bluest galaxies ( ) in our samples, independent of dust law and consistent with results for a z = 7.045 galaxy. Such elevated values of would have important consequences, indicating that f esc cannot be in excess of 13% for standard assumptions about the faint-end cut-off to the LF and the clumping factor.

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