The γ-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.