The Guaranteed
Gamma-Ray Background

The gamma-ray background is a constant flux of gamma rays arriving uniformly from all directions. Persumably extragalactic in origin, the gamma-ray background encodes information about the highest-energy processes in the universe. Many possible sources have been proposed, some more exotic than others. However, two classes of sources are guaranteed to have some contribution: normal galaxies, and active galaxies.

Normal galaxies and active galaxies are the only two classes of extragalactic gamma-ray sources that have been associated with known and understood astrophysical systems. Many active galaxies and two normal galaxies (our own Milky Way and the LMC) have been seen in gamma-rays. Many more fainter and more distant similar sources are certain to exist, and they are guaranteed to make some contribution to the gamma-ray background.

We have calculated the contribution of normal galaxies to the gamma-ray background using a new model, based on observations of the cosmic star formation rate to caculate the cosmic ray flux and gas content of typical galaxies at every redshift, and from these their mean gamma-ray flux. We have added this result to the contribution of Blazars as given by the Stecker-Salamon model. We found 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. This two-component model will be testable by GLAST, which will resolve out a large part of the blazar contribution, and might detect a predicted feature due to the normal-galaxy component.

Reference: Pavlidou, V., & Fields, B. D. 2002, ApJ, 575, L5
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