The Lyman-continuum photon production efficiency ξ ION OF z∼ 4–5 galaxies from IRAC-based Hα measurements : implications for the escape fraction and cosmic reionization

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 $z\gt 6$, significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency ${\xi }_{\mathrm{ion}}$ and the escape fraction fesc. 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 ${\xi }_{\mathrm{ion}}$ for the $z\geqslant 4$ galaxy population. We find $\,{\mathrm{log}}_{10}\,{\xi }_{\mathrm{ion}}/[\mathrm{Hz}\,{\mathrm{erg}}^{-1}]$ 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 ${\xi }_{\mathrm{ion}}$'s in reionization models, with a slight preference for higher ${\xi }_{\mathrm{ion}}$'s (by ∼0.1 dex) adopting the SMC dust law. High values of ${\xi }_{\mathrm{ion}}$ (∼25.5–25.8 dex) are derived for the bluest galaxies ($\beta \lt -2.3$) in our samples, independent of dust law and consistent with results for a z = 7.045 galaxy. Such elevated values of ${\xi }_{\mathrm{ion}}$ would have important consequences, indicating that fesc cannot be in excess of 13% for standard assumptions about the faint-end cut-off to the LF and the clumping factor.