The Universe Viewed in Gamma-Rays 1 The Guaranteed Gamma-ray Background


The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodes unique information about high-energy processes in the universe. The systems certain to make some contribution to the EGRB are blazars and normal galaxies. We evaluate their combined contribution to the background using the Stecker-Salamon model for the blazar component and a new calculation for the normal galaxy component. Assuming that most of the gamma-ray emission from normal galaxies is due to cosmic-ray interactions with diffuse gas, we use cosmic star formation rate observations to follow the evolution of the cosmic ray flux and the gas content of normal galaxies. We find that normal galaxies are responsible for a significant portion ( ∼ 1/3) of the EGRB near 1 GeV, but make a smaller contribution at other energies. The combined spectrum of this 2-component model is an excellent fit to the EGRET observations of the EGRB spectrum. Finally, we discuss a series of observational tests for the two-component model which can be performed by future gamma-ray observatories such as GLAST. 1

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