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 yr of gamma-ray data taken by the 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 similar to 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 2.2-2.3. 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.