Star-Forming Galaxies in the Diffuse Background: Why a Steep Spectrum Confines Them to Low Energies

Star-forming galaxies emit gamma rays from cosmic-ray interactions in their interstellar media, and the Fermi Large Area Telescope collaboration has estimated their collective contribution to the isotropic background. This paper recomputes that contribution's energy dependence from the published central intensity at 1 GeV and the steep, pion-decay-like spectrum of the population, with the published uncertainty band carried through. It finds fraction at 1GeV = 0.12; fraction at 10GeV = 0.05; fraction at 100GeV = 0.0209. Even at the top of the published band, star-forming galaxies cannot supply the high-energy background; at the bottom, they are a minor correction even at low energy. The study is simulated and computed, not measured.