Search for 511 keV electron-positron annihilation radiation from mildly active galaxies using the HEAO 3 gamma-ray spectrometer

The HEAO 3 spacecraft's gamma-ray spectrometer has been used to obtain upper limits to the 511-keV electron-positron annihilation line flux from seven galaxies: M 81/M 82, NGC 4278, M 104, NGC 6500, NGC 2911, and NGC 262. These appear to contain scaled-up versions of the radio source in the galactic center, and their nonthermal activity appears to be confined to the central regions, implying that energetic particles produced in these nuclei do not escape into the low density regions of interstellar or intergalactic space. On the basis of the scaling law adopted, six of these seven galaxies should have been detected above the upper limits obtained for narrow 511 keV emission. Explanations are adduced which account for the scaling law's failure to predict the correct level of radiation annihilation.