Diffuse gamma-ray emission from starburst galaxies and M31

We present a search for high-energy gamma-ray emission from nine nearby starburst galaxies and M31 with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO). Although the diffuse gamma-ray emission from starburst galaxies was suspected to be detectable, we find no emission from NGC 253, from M82, or from the average of all nine galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the averaged survey observations is 1.8 x 10(-8) photons cm(-2) s(-1). From a model of the expected radio and gamma-ray emission, we find that the magnetic field in the nuclei of these galaxies is greater than 25 mu G, and the ratio of proton and electron densities is less than 400. The EGRET limits indicate that the rate of massive star formation in the survey galaxies is only about an order of magnitude higher than in the Milky Way. The upper limit to the gamma-ray flux above 100 MeV for M31 is 1.6 x 10(-8) photons cm-2 s(-1). At the distance of M31, the Milky Way flux would be over twice this value, indicating higher gamma-ray emissivities in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher than that in M31, our null detection of M31 supports the theory of the supernova origin of cosmic rays in galaxies.