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

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 z > 6, significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency xi(ion) 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 Ha line has on the observed IRAC fluxes. By computing a LyC photon production rate from the implied Ha 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 xion for the z >= 4 galaxy population. We find log(10) xi(ion)/[Hz erg(-1)] to have a mean value of 25.27(-0.03)(+0.03) and 25.34(-0.02)(+0.02) for sub-L* z = 4-5 galaxies adopting Calzetti and SMC dust laws, respectively. Reassuringly, both derived values are consistent with the standard assumed xi(ion)'s in reionization models, with a slight preference for higher xi(ion)'s (by similar to 0.1 dex) adopting the SMC dust law. High values of xi(ion) (similar to 25.5-25.8 dex) are derived for the bluest galaxies (beta < -2.3) in our samples, independent of dust law and consistent with results for a z = 7.045 galaxy. Such elevated values of xion 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.