The Gamma-Ray Luminosity of Spiral Galaxies, its Evolution and its Contribution to the Diffuse Background above 100 MeV

The contribution of normal spiral galaxies to the high galactic latitude gamma-ray background greater than 100 MeV was examined in the light of the estimates of its flux from the SAS-2 measurements. The gamma ray luminosity of each object is inferred from the known Milky Way value normalized to the corresponding optical quantity. Several possibilities were considered for the responsible physical production mechanism both diffuse and localized; these were then set to evolve with the galactic age according to 3 well-known evolutionary models. A final spacetime integration led to results, expressed as a fraction of the measured background. It is seen that the model presented can play an important role in the region greater than 100 MeV where the information on the spectral shape of the radiation is still very poor.