Modeling the High-energy Ionizing Output from Simple Stellar and X-Ray Binary 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 ultraluminous X-ray sources 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 λ1640,4686, [Ne v] λ3426,14.3 μm, and [O iv] 25.9 μm 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 (∼0.1 Z ⊙), 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 we discuss how the ionization signatures compare to models for sources such as intermediate-mass black holes.

EMPRESS. XIV. Strong High-ionization Lines of Young Galaxies at z = 0–8: Ionizing Spectra Consistent with the Intermediate-mass Black Holes with M BH ∼ 103–106 M ⊙

“…The hard ionizing radiation may be caused by BHs (e.g., Olivier et al 2021;Simmonds et al 2021;Umeda et al 2022). There is a possibility that X-ray binaries (XRBs) with stellar-mass BHs produce the hard ionizing radiation (Schaerer et al 2019;Garofali et al 2024), although a famous dwarf galaxy, SBS 0335-052E, which shows strong He II λ4686 emission, has an X-ray luminosity lower than expected from XRBs (Kehrig et al 2018;Saxena et al 2020; see also Thuan et al 2004). Ultraluminous X-rays (ULXs) have been proposed to explain the observed [Ne V] λ3426 for some dwarf galaxies (Simmonds et al 2021;Umeda et al 2022).…”

“…There are other ionizing spectra modeling techniques developed by Olivier et al (2021), Simmonds et al (2021), Garofali et al (2024), andIzotov et al (2021). Olivier et al (2021) use the sum of stellar synthesis models and blackbody radiation.…”

“…Simmonds et al (2021) utilize observationally constrained spectra of ULXs and BPASS binary models. Garofali et al (2024) utilize the sum of BPASS models and XRB spectra. Izotov et al (2021) use nonthermal power-law radiation, BPASS single models, and STARBURST99 models.…”

“…Izotov et al (2021) use nonthermal power-law radiation, BPASS single models, and STARBURST99 models. Because our spectra free the spectral index and amplitude of the power-law component, the SEDs of our models include those of Olivier et al (2021), Simmonds et al (2021), Garofali et al (2024), andIzotov et al (2021).…”

No Top-heavy Stellar Initial Mass Function Needed: The Ionizing Radiation of GS9422 Can Be Powered by a Mixture of an Active Galactic Nucleus and Stars

“…The JWST/NIRSpec observation of GS9422 was acquired by the JWST Advanced Deep Extragalactic Survey (JADES; Eisenstein et al 2023) in four grating modes, G140M/ F070LP, G235/F170LP, G395M/F290LP, and G395H/ F290LP, with 7 hr integration for each, and in the lowresolution PRISM/CLEAR mode with 28 hr integration. Cameron et al (2023a) highlight two main observational constraints that disfavor typical ionizing sources; a UV turnover at λ ≈ 1430 Å combined with a Balmer jump suggest strong two-photon nebular continuum emission and high neutral hydrogen absorption for Lyα; and the detected He II emission lines are too faint for AGN, or X-ray binaries (e.g., Garofali et al 2024). A stellar population with an exotic, top-heavy IMF including 10-30x more young massive stars/Wolf-Rayet stars (WR) stars than the typical IMF can solve the above two problems.…”