  We investigate the nebular emission produced by young stellar populations
using the new GALSEVN model based on the combination of the SEVN
population-synthesis code including binary-star processes and the GALAXEV code
for the spectral evolution of stellar populations. Photoionization calculations
performed with the CLOUDY code confirm that accounting for binary-star
processes strongly influences the predicted emission-line properties of young
galaxies. In particular, we find that our model naturally reproduces the strong
HeII/Hb ratios commonly observed at high Hb equivalent widths in metal-poor,
actively star-forming galaxies, which have proven challenging to reproduce
using previous models. Including bursty star formation histories broadens the
agreement with observations, while the most extreme HeII equivalent widths can
be reproduced by models dominated by massive stars. GALSEVN also enables us to
compute, for the first time in a way physically consistent with stellar
emission, the emission from accretion discs of X-ray binaries (XRBs) and
radiative shocks driven by stellar winds and supernova explosions. We find that
these contributions are unlikely to prominently affect the predicted HeII/Hb
ratio, and that previous claims of a significant contribution by XRBs to the
luminosities of high-ionization lines are based on models predicting improbably
high ratios of X-ray luminosity to star formation rate, inconsistent with the
observed average luminosity function of XRBs in nearby galaxies. The results
presented here provide a solid basis for a more comprehensive investigation of
the physical properties of observed galaxies with GALSEVN using Bayesian
inference.
