Semi-analytic forecasts for JWST -- III. Intrinsic production efficiency of Lyman-continuum radiation

TL;DR

This paper predicts the Lyman-continuum production efficiency of galaxies during the Epoch of Reionization using semi-analytic models combined with stellar population synthesis, highlighting the impact of binary stars and galaxy properties on ionizing photon output.

Contribution

It introduces a semi-analytic framework to predict ionizing photon production efficiency and explores the effects of stellar populations and feedback on reionization sources.

Findings

Binary star models increase ionizing radiation predictions by a factor of two.

UV-faint, low-mass galaxies have higher $\xi_{ m ion}$ than massive ones.

Weak evolution of $\xi_{ m ion}$ with cosmic time from $z=12$ to $4$.

Abstract

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/.