COMPACT RADIO-SOURCES IN THE STARBURST GALAXY M82 AND THE SIGMA-D RELATION FOR SUPERNOVA-REMNANTS

We have obtained an 8.4 GHz VLA A-array map of the starburst galaxy M82 with a resolution (FWHM) approximately 0.182''. About 50 compact radio sources in the central region of M82 were detected with a peak surface brightness greater than or similarto 10(17) W Hz-1 m-2 sr-1. Comparison with previous observations shows that most sources are declining in flux. Three previously visible sources have faded into the background of our map (less than or similar to 0.2 mJy beam-1), while a few sources, including the second and third brightest radio sources in M82, may have increased slightly in flux over the last decade. No new radio supernova was found. The birth rate of the compact radio sources is estimated to be 0.11 +/- 0.05 yr-1. We attribute the population of such bright, small supernova remnants in M82 to the high pressure in the central region that can truncate the mass loss during a red supergiant phase or allow dense ionized clouds to be present. The compact radio sources obey a SIGMA(radio surface brightness) - D(diameter) relation which is remarkably similar to that followed by supernova remnants in the Galaxy and the Magellanic Clouds and by two of the strongest known extragalactic radio supernovae: SN 1986J and SN 1979C. A least-squares fit to the SNR data gives: SIGMA8.4 GHz (W Hz-1 m-2 sr-1) = 4.4 x 10(-16) D(pc)-3.5 +/- 0.1 covering seven orders of magnitude in SIGMA. Possible selection effects are discussed and a theoretical discussion of the correlation is presented.