The γ-Ray Emission of Star-forming Galaxies
M. Ajello, M. Di Mauro, V. S. Paliya, S. Garrappa
Abstract
Abstract A majority of the γ -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 γ -ray background and the IceCube astrophysical neutrino flux. Using roughly 10 yr of γ -ray data taken by the Fermi Large Area Telescope, in this study we constrain the γ -ray properties of star-forming galaxies. We report the detection of 11 bona fide γ -ray-emitting galaxies and 2 candidates. Moreover, we show that the cumulative γ -ray emission of below-threshold galaxies is also significantly detected at ∼5 σ confidence. The γ -ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8–1000 μ 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 γ -ray background and the IceCube neutrino flux, respectively.