The $\gamma$-ray Emission of Star-Forming Galaxies
  A majority of the $\gamma$-ray emission from star-forming galaxies is
generated by the interaction of high-energy cosmic rays with the interstellar
gas and radiation fields. Star-forming galaxies are expected to contribute to
both the extragalactic $\gamma$-ray background and the IceCube astrophysical
neutrino flux. Using roughly 10\,years of $\gamma$-ray data taken by the {\it
Fermi} Large Area Telescope, in this study we constrain the $\gamma$-ray
properties of star-forming galaxies. We report the detection of 11 bona-fide
$\gamma$-ray emitting galaxies and 2 candidates. Moreover, we show that the
cumulative $\gamma$-ray emission of below-threshold galaxies is also
significantly detected at $\sim$5\,$\sigma$ confidence. The $\gamma$-ray
luminosity of resolved and unresolved galaxies is found to correlate with the
total (8-1000\,$\mu$m) infrared luminosity as previously determined. Above
1\,GeV, the spectral energy distribution of resolved and unresolved galaxies is
found to be compatible with a power law with a photon index of
$\approx2.2-2.3$.
  Finally, we find that star-forming galaxies account for roughly 5\,\% and
3\,\% of the extragalactic $\gamma$-ray background and the IceCube neutrino
flux, respectively.
