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) beam. The observations confirm that most of the discrete sources are supernova remnants. Several show complete shells, and many more show structure consistent with partial shells. We have measured the sizes, flux densities and radio brightnesses of 23 of the sources, and show that they follow a surface brightness/diameter relation consistent with that found in the LMC and Galactic remnants. The cumulative number as a function of diameter appears to increase linearly to diameters of at least 3 pc, and indicates a supernova rate of 0.05 per year if the shells are expanding at $$5000 \ {\rm km \ s}^{-1}$$. 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 secular flux decrease of $$\sim 0.5$$ per cent for typical remnants in M82. None of the discrete sources appears to show inverted-spectrum unresolved cores or radio jets which we could unambiguously attribute to the central source of an AGN.