A STUDY OF M31, M87, NGC 253, AND M82 IN HIGH-ENERGY GAMMA-RAYS

The data from the Energetic Gamma Ray Experiment Telescope (EGRET) all -sky survey are examined for emission from the nearby galaxies M31, M8 7, NGC 253, and M82 in the high-energy (E > 30 MeV) gamma-ray range. N o significant emission is observed from any of these galaxies. The der ived upper limits for all four galaxies are consistent with that expec ted from cosmic-ray interactions. For M87, the combination of the high -energy gamma-ray and radio data point to a lower limit of 7 muG for t he magnetic field in the disk and 4 muG for the magnetic field in the halo, consistent with equipartition arguments. A study of NGC 253 and M82 show that EGRET observations do not sufficiently constrain the mea n cosmic-ray energy density to provide more details on the cosmic-ray distribution in starburst galaxies.