Finding He II: Testing Novel Models of Binary Populations across Cosmic Time

TL;DR

This work tackles the puzzle of hard ionizing spectra in high-redshift galaxies, where photons with energies above $54.4$ eV are needed to produce He II emission that single-star models struggle to supply. It compares three population-synthesis frameworks—Starburst99 (single stars), BPASS (binary evolution), and Gotberg2019 stripped-star models appended to Starburst99—across a grid of ages and metallicities, then feeds the spectra into Cloudy to predict nebular continua and line ratios in rest-UV–optical data. The results show that binary treatment materially alters the predicted ionizing spectrum and the He II $1640$/Hβ ratio, with stripped stars boosting He II flux after ~10 Myr and BPASS introducing additional hard-photon channels, including potential quasi-chemically homogeneous evolution effects. Overall, He II $1640$ serves as a diagnostic to constrain massive-star populations at high redshift and to test for phenomena such as the helium-star desert, thereby guiding model improvements and interpretation of JWST-era observations.

Abstract

Our understanding of massive stars remains incomplete. Many high-z galaxies and nearby analogs exhibit strong He II emission, indicating an abundance of photons with energies >54.4 eV that standard single-star population models cannot explain. Recent studies show that binary evolution and non-solar abundance patterns are required to explain the distinct spectra of high-z galaxies observed by JWST. However, treatments of these properties vary drastically between models. We present the first results from a comparison of models with different treatments of binaries, including BPASS and novel stripped star models, with rest-UV-optical spectra of high-z galaxies' local analogs. This type of investigation can provide insights into which aspects of binary evolution are important to reproduce observations and identify priorities in ongoing efforts to improve models. By constraining the properties of massive stars at high redshift, we can learn about the processes at play in high-z galaxies and massive star evolution more broadly.

Figures (3)

• Figure 1: Estimates for hydrogen- ($Q_0$, left) and helium-ionizing ($Q_2$, right) emission rates from Hovis-Afflerbach2025. The total rates are shown as a sum of rates for single stars (light yellow), low-mass stripped stars ($M < 8.5\ M_\odot$, medium yellow), and higher-mass stripped stars ($M > 8.5\ M_\odot$, dark yellow). Stripped stars dominate $Q_2$, resulting in a significantly higher rate if there is no helium star desert (left column) than if there is (middle column).
• Figure 2: Comparison of metallicity (left) and age (right) for Starburst99 continuum fits compared with BPASS (purple) and Starburst99 with stripped stars (green). While the BPASS results generally agree with those from Starburst99, the stripped star results are virtually identical.
• Figure 3: He II $\lambda 1640$/H$\beta$ as a function of time after a starburst, from Cloudy runs using incident spectra from Starburst99 (circles, dashed line), Starburst99 plus Gotberg2019 stripped stars (stars, solid line), and BPASS (diamonds, dotted line). The four metallicities in the model grid are indicated by color. Both BPASS and stripped stars boost He II $\lambda 1640$/H$\beta$ significantly beginning at 10 Myr.