Blue galaxies: modelling nebular He II emission in high redshift
  galaxies
  Using cosmological simulations to make useful, scientifically relevant
emission line predictions is a relatively new and rapidly evolving field.
However, nebular emission lines have been particularly challenging to model
because they are extremely sensitive to the local photoionization balance,
which can be driven by a spatially dispersed distribution of stars amidst an
inhomogeneous absorbing medium of dust and gas. As such, several unmodeled
mysteries in observed emission line patterns exist in the literature. For
example, there is some question as to why He II $\lambda 4686$/H$\beta$ ratios
in observations of lower-metallicity dwarf galaxies tend to be higher than
model predictions. Since hydrodynamic cosmological simulations are best suited
to this mass and metallicity regime, this question presents a good test case
for the development of a robust emission line modeling pipeline. The pipeline
described in this work can model a process that produces high He II $\lambda
4686$/H$\beta$ ratios and eliminate some of the modeling discrepancy for ratios
below 3% without including AGNs, X-ray binaries, high mass binaries, or a
top-heavy stellar initial mass function. These ratios are found to be more
sensitive to the presence of 15 Myr or longer gaps in the star formation
histories than to extraordinary ionization parameters or specific star
formation rates. This work also charts a path forward for the next generation
of nebular emission line modeling studies.
