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 measureable in z = 4 -5 galaxies based on the impact the H \alpha line has on the observed IRAC fluxes .
By computing a LyC photon production rate from the implied H \alpha 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 \xi _ { ion } for the z \geq 4 galaxy population .
We find \log _ { 10 } \xi _ { ion } / [ \textrm { Hz ergs } ^ { -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 standardly assumed \xi _ { ion } ’ s in reionization models , with a slight preference for higher \xi _ { ion } ’ s ( by \sim 0.1 dex ) adopting the SMC dust law .
High values of \xi _ { ion } ( \sim 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 \xi _ { ion } would have important consequences , indicating that f _ { esc } can not be in excess of 13 % for standard assumptions about the faint-end cut-off to the LF and the clumping factor .