The Guaranteed Gamma-Ray Background

The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodesunique information about high-energy processes in the universe. Numeroussources for the EGRB have been proposed, but the two systems which are certainto make some contribution are active galaxies (blazars) as well as normalgalaxies. In this paper, we evaluate the contribution to the background fromboth sources. The active galaxy contribution arises from unresolved blazars. Wecompute this contribution using the Stecker-Salamon model. For normal galaxies,the emission is due to cosmic-ray interactions with diffuse gas. Our keyassumption here is that the cosmic-ray flux in a galaxy is proportional to thesupernova rate and thus the massive star formation rate, quantifiedobservationally by the cosmic star formation rate (CSFR). In addition, theexistence of stars today requires a considerably higher ISM mass in the past.Using the CSFR to compute both these effects, we find that normal galaxies areresponsible for a significant portion (\sim 1/3) of the EGRB near 1 GeV, butmake a smaller contribution at other energies. Finally, we present a "minimal"two-component model which includes contributions from both normal galaxies andblazars. We show that the spectrum of the diffuse radiation is a key constrainton this model: while neither the blazar spectra, nor the galactic spectra, areseparately optimal fits to the observed spectrum, the combined emissionprovides an excellent fit. We close by noting key observational tests of thistwo-component model, which can be probed by future gamma-ray observatories suchas GLAST.Comment: 11 pages, 1 figure, uses AASTeX. To appear in ApJ