Can Supersoft X-ray Sources Power Nebular He&#8239-II Emission? Modeling Predictions and Observational Clues
Dian Puspita Triani, Caroline Erickson, Atanas Mitrev, Peixin Zhu, Rosanne Di Stefano

Center for Astrophysics Harvard Smithsonian

Abstract

Nebular He&#8239-II&#8239-&#955-4686 emission remains one of the key diagnostics of hard ionizing radiation in galaxies, yet its origin, particularly in star-forming systems, remains uncertain. Supersoft X-ray sources (SSSs) are theoretically capable of producing He&#8314--ionizing photons but have often been neglected as contributors to He&#8239-II emission. In this talk, I present our recent modeling of SSSs using blackbody spectra and photoionization calculations to assess how the number and luminosity distribution of SSSs affect He&#8239-II ionization. We compare these predictions to observed He&#8239-II/H&#946- ratios in galaxies to evaluate whether SSSs can account for the required ionizing flux.

I also highlight two student-led investigations that provide new observational constraints. First, using resolved data from M51, we examine the environments of known SSSs and find that the most luminous SSSs are embedded in dense gas in the central regions, favorable conditions for producing nebular He&#8239-II. Second, a cross-matched analysis of SDSS He&#8239-II emitters and XMM-Newton X-ray sources reveals that He&#8239-II and X-ray luminosities are tightly correlated across both star-forming galaxies and AGN. Interestingly, star-forming galaxies exhibit a higher He&#8239-II-to-X-ray luminosity ratio (LHeII / L&#8339-) and softer X-ray spectra compared to AGN, suggesting that softer, lower-luminosity X-ray sources may be more efficient He&#8239-II producers in these systems.

Keywords: Interstellar medium, X-ray, photoionization

Topic: Galaxies and Cosmology