2025ApJ...993..117T

Nebular spectral lines provide insight into the properties of the interstellar medium and the ionizing radiation within galaxies. The presence of high-energy ionization lines such as He II indicates the existence of ionizing photons with energies exceeding the second ionization energy of helium (54 eV). There is an enigma surrounding the origin of these lines observed in star-forming galaxies because stellar ionization cannot account for such high energy emission. This paper proposes that supersoft X-ray sources (SSSs) may produce the He II ionization lines in star-forming galaxies. We model the spectra of SSSs using blackbody radiation and add them to the young stellar population spectra to represent the overall spectra of galaxies. Using a photoionization model, we predict the resulting He IIλ4686 and Hβ line fluxes and inspect the contribution of SSSs to the elevation of the He IIλ4686 /Hβ ratio in star-forming galaxies, both at low and high redshifts. We find that incorporating a blackbody with temperatures between kT = 10-100 eV can boost the He IIλ4686 /Hβ line ratio to the levels observed in local galaxies by SDSS and in early galaxies by NIRSpec. This blackbody temperature range aligns with the observed temperatures of SSSs. The number of SSSs in spiral galaxies listed in Chandra catalogs, and our estimates of the total population, confirms that SSSs are promising candidates for the source of the He II ionization.