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

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, ξ 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 ξ ion on galaxy properties and cosmic epoch from z = 4-10. We then explore the sensitivity of the production rate of ionizing photons,Ṅ 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 ξ 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 ∼ 12-4 at fixed rest-UV luminosity. We provide predictions ofṄ ion as a function of M 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/.

The Epoch of Reionization in Warm Dark Matter Scenarios

“…Yung et al (2020) developed a semi-analytic approach with a Chabrier IMF and compared their results with previous studies, including the hydrodynamic simulations presented in Wilkins et al (2016). They found that ξ ion was nearly unaffected by the change from Chabrier to Salpeter IMF [52]. Despite the different IMF, also their values agree with our fiducial ξ ion = 10 25.2 Hz/erg assumption.…”

“…Finally, Eldridge et al (2017) demonstrated that ξ ion is sensitive to variation in the upper end of the stellar IMF. In particular, they highlighed a lower photon production efficiency in IMF with power law slope extended up to 100 M with respect to similar models extended up to 300 M [52,55]. These evaluations lead us to neglect the variations induced on ξ ion by the IMF.…”

“…We note that log(ξ ion f esc ) increases with M hm : a larger escape fraction and/or UV photons production efficiency are needed to complete the Reionization process in WDM scenarios. However, the quantity ξ ion is better constrained than f esc , so we assume from the literature a fiducial value of log(ξ ion /(erg −1 Hz)) = 25.2 [34,37,51,52], as expected from a low metallicity single-star population. This value is coherent with the Salpeter IMF assumed in the semi-analytic model [50,53] (see Section 4.3.2).…”

“…Some studies predict the value of LyC production efficiency using stellar population synthesis (SPS) codes, which combine evolution and atmosphere models, for a given IMF. In particular, uncertainties on ξ ion are related to treatment of convection, rotation, and evolution of binary stars [52]. Wilkins et al (2016) noted that changes in the low-mass end of the Salpeter IMF have a minimal effect on ξ ion , in five SPS models [53].…”

An Empirical Representation of a Physical Model for the ISM [C ii], CO, and [C i] Emission at Redshift 1 ≤ z ≤ 9

“…This feature makes Santa Cruz SAM particularly suitable for ISM line emission predictions. The SFR, stellar mass, cold gas mass, and other predictions of the Santa Cruz SAM have been shown to be broadly consistent with observations from the local universe to z ∼ 10 ( Popping et al 2014b;Yung et al 2019aYung et al , 2019bYung et al , 2020aYung et al , 2020bSomerville et al 2021).…”