  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 hard
ionizing photons, because the second ionization energy of helium is high (54
eV). The enigma surrounding the origin of these lines observed in star-forming
galaxies persists, as stellar ionization cannot account for such high energy
emission. This paper proposes that supersoft X-ray sources (SSS) may produce
these high energy ionization lines in star-forming galaxies. We model the
spectra of such sources using blackbody radiation and then add the blackbody to
stellar population spectra to represent the overall spectra of galaxies. We
then use a photoionization model to predict the resulting emission lines and
compare the contribution of SSS to the observation of highly ionized lines in
star-forming galaxies, both at low and high redshifts. We find that
incorporating a blackbody with temperatures between kT = 20-100eV can boost the
He II emission line ratio to the observed level. The blackbody temperature
range aligns with the observed temperatures of the SSSs. The number of SSSs in
spiral galaxies listed in Chandra catalogues, and our estimates of the total
population, confirms that SSSs are promising candidates for the source of the
He II ionization.
