Semi-analytic forecasts for JWST -- III. Intrinsic production efficiency
  of Lyman-continuum radiation
  The James Webb Space Telescope (JWST) is expected to enable transformational
progress in studying galaxy populations in the very early Universe, during the
Epoch of Reionization (EoR). A critical parameter for understanding the sources
that reionized the Universe is the Lyman-continuum production efficiency,
$\xi_{\rm ion}$, defined as the rate of production of ionizing photons divided
by the intrinsic UV luminosity. In this work, we combine self-consistent star
formation and chemical enrichment histories predicted by semi-analytic models
of galaxy formation with stellar population synthesis (SPS) models to predict
the expected dependence of $\xi_{\rm ion}$ on galaxy properties and cosmic
epoch from $z=4-10$. We then explore the sensitivity of the production rate of
ionizing photons, $\dot{N}_{\rm ion}$, to the choice of SPS model and the
treatment of stellar feedback in our galaxy formation model. We compare our
results to those of other simulations, constraints from empirical models, and
observations. We find that adopting SPS models that include binary stars
predict about a factor of two more ionizing radiation than models that only
assume single stellar populations. We find that UV-faint, low-mass galaxies
have values of $\xi_{\rm ion}$ about 0.25 dex higher than those of more massive
galaxies, but find weak evolution with cosmic time, about 0.2 dex from $z\sim
12-4$ at fixed rest-UV luminosity. We provide predictions of $\dot{N}_{\rm
ion}$ as a function of $M_{\rm h}$ and a number of other galaxy properties. All
results presented in this work are available at
https://www.simonsfoundation.org/semi-analytic-forecasts-for-jwst/.
