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>6, significant uncertainties remain regarding the
Lyman-continuum (LyC) photon production efficiency xi_ion and the escape
fraction f_esc. However, dramatic progress can be made in assessing the
impact of z>~6 galaxies on the reionization of the universe, using the Halpha
fluxes inferred from z=4-5 galaxies based on their IRAC broad-band fluxes.
Here, we provide the first-ever direct estimates of the LyC photon production
efficiency xi_ion for z>~4 galaxies, by comparing the LyC photons implied by
the inferred H$\alpha$ luminosities for a $z=4$-5 sample with the
dust-corrected UV-continuum luminosities. We find log_10 xi_ion/Hz / ergs
to have a mean value of 25.28_-0.09^+0.10 and 25.34_-0.08^+0.09 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 ~0.1 dex) adopting the SMC dust law. A modest ~0.03-dex increase
in these estimates would result if the escape fraction for ionizing photons is
non-zero and galaxies dominate the ionizing emissivity at z~4.4. High values of
xi_ion (~25.5-25.9 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 cannot be in excess of 13% unless the galaxy UV
luminosity function does not extend down to -13 mag or the clumping factor is
greater than 3. A low escape fraction would fit well with the low rate of
Lyman-continuum leakage observed at z~3.
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