Nebular Continuum and Line Emission in Stellar Population Synthesis
  Models
  Accounting for nebular emission when modeling galaxy spectral energy
distributions (SEDs) is important, as both line and continuum emission can
contribute significantly to the total observed flux. In this work, we present a
new nebular emission model integrated within the Flexible Stellar Population
Synthesis code that computes the total line and continuum emission for complex
stellar populations using the photoionization code Cloudy. The self-consistent
coupling of the nebular emission to the matched ionizing spectrum produces
emission line intensities that correctly scale with the stellar population as a
function of age and metallicity. This more complete model of galaxy SEDs will
improve estimates of global gas properties derived with diagnostic diagrams,
star formation rates based on H$\alpha$, and stellar masses derived from NIR
broadband photometry. Our models agree well with results from other
photoionization models and are able to reproduce observed emission from H II
regions and star-forming galaxies. Our models show improved agreement with the
observed H II regions in the Ne III/O II plane and show satisfactory agreement
with He II emission from $z=2$ galaxies when including rotating stellar models.
Models including post-asymptotic giant branch stars are able to reproduce line
ratios consistent with low-ionization emission regions (LIERs).
