THE STRUCTURE OF YOUNG SUPERNOVA-REMNANTS IN M82

We have used the enhanced MERLIN at 5 GHz to image the central 700 pc of the nearby starburst galaxy M82. We detect in excess of 40 discrete sources, all of which are resolved by the MERLIN 50-mas (0.75-pc) bea m. The observations confirm that most of the discrete sources are supe rnova remnants. Several show complete shells, and many more show struc ture consistent with partial shells. We have measured the sizes, flux densities and radio brightnesses of 23 of the sources, and show that t hey follow a surface brightness/diameter relation consistent with that found in the LMC and Galactic remnants. The cumulative number as a fu nction of diameter appears to increase linearly to diameters of at lea st 3 pc, and indicates a supernova rate of 0.05 per year if the shells are expanding at 5000 km s(-1). The flux densities of the SNR in M82 and the LMC decrease approximately inversely as the diameters. Hence, if we assume that diameter is related to age, this would imply a secul ar flux decrease of similar to 0.5 per cent for typical remnants in M8 2. None of the discrete sources appears to show inverted-spectrum unre solved cores or radio jets which we could unambiguously attribute to t he central source of an AGN.