DIFFUSE GAMMA-RAY EMISSION FROM THE STARBURST GALAXY NGC 253

The starburst galaxy NGC 253 was observed with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) satellite. We obtain a 2 tr upper limit to the )'-ray emission above 100 MeV of 8 × 10 s photons cm 2 s t . Because of their large gas column densities and supernova rates, nearby starburst galaxies were predicted to have )'-ray fluxes detectable by EGRET. Our nondetection of )'-rays from NGC 253 motivates us to reexamine in detail the premise of supernova acceleration of cosmic rays and the effect of enhanced cloud densities, photon densities, and magnetic fields on the high-energy spectra of galaxies. By modeling the expected )'-ray and synchrotron spectra from NGC 253, we find that up to 20% of the energy from supernovae is transferred to cosmic rays in the starburst, which is consistent with supernova acceleration models. Our calculations match the EGRET and radio data well with a supernova rate of 0.08 yr 1, a magnetic field B > 5 x 10 5 G, a density n ~ 300 cm -3, a photon density Uph ~ 200 eV cm 3, and an escape timescale t 0 < 10 Myr. Subject headings: cosmic rays - galaxies: individual (NGC 253) - galaxies: starburst gamma rays: observations gamma rays: theory stars: statistics