The Star-Forming Galaxy Contribution to the Cosmic MeV and GeV Gamma-Ray
  Background
  While star-forming galaxies could be major contributors to the cosmic GeV
$\gamma$-ray background, they are expected to be MeV-dim because of the "pion
bump" falling off below ~100 MeV. However, there are very few observations of
galaxies in the MeV range, and other emission processes could be present. We
investigate the MeV background from star-forming galaxies by running one-zone
models of cosmic ray populations, including Inverse Compton and bremsstrahlung,
as well as nuclear lines (including $^{26}$Al), emission from core-collapse
supernovae, and positron annihilation emission, in addition to the pionic
emission. We use the Milky Way and M82 as templates of normal and starburst
galaxies, and compare our models to radio and GeV--TeV $\gamma$-ray data. We
find that (1) higher gas densities in high-z normal galaxies lead to a strong
pion bump, (2) starbursts may have significant MeV emission if their magnetic
field strengths are low, and (3) cascades can contribute to the MeV emission of
starbursts if they emit mainly hadronic $\gamma$-rays. Our fiducial model
predicts that most of the unresolved GeV background is from star-forming
galaxies, but this prediction is uncertain by an order of magnitude. About ~2%
of the claimed 1 MeV background is diffuse emission from star-forming galaxies;
we place a firm upper limit of <~10% based on the spectral shape of the
background. The star-formation contribution is constrained to be small, because
its spectrum is peaked, while the observed background is steeply falling with
energy through the MeV-GeV range.
