Modeling the High-Energy Ionizing Output from Simple Stellar and X-ray Binary Populations

TL;DR

This paper develops a method to model the combined ionizing effects of stellar populations and X-ray sources, revealing their impact on nebular emission lines and providing diagnostics for JWST observations.

Contribution

It introduces a new composite spectral energy distribution model including ULXs and stars, and provides a grid of nebular emission predictions as a function of age and metallicity.

Findings

Adding SXP increases nebular line intensities by at least a factor of two.

The effect is strongest for starbursts older than 10 Myr at low metallicity.

The results enable new diagnostics for JWST to identify composite populations.

Abstract

We present a methodology for modeling the joint ionizing impact due to a "simple X-ray population" (SXP) and its corresponding simple stellar population (SSP), where "simple" refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultra-luminous X-ray sources (ULXs) and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population, and correspondingly increases the intensity of nebular lines such as He II $\lambda$1640,4686, [Ne V] $\lambda$3426,14.3$\mu$m, and [O IV] 25.9$\mu$m by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales $>$ 10 Myr and at low metallicities ($\sim$ 0.1 $Z_{\odot}$), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and discuss how the ionization signatures compare to models for sources such as intermediate mass black holes.