Supernova remnants in the starburst galaxy M82

We have used the enhanced MERLIN at 1.5 and 5 GHz to image the central 700pc of the nearby starburst galaxy M82. Of order 40 discrete sources are detected and it appears that most of these sources are supernova remnants. Not only do many show shell structure and have a non-thermal radio spectrum, but they also follow a surface brightness/diameter relation consistent with that found in the LMC and Galactic supernova remnants. The detected M82 remnants are more compact and brighter than Galactic remnants which implies that they must be less than a few hundred years old and hence supernova rates are of order 0.05 per year. The 1.5 GHz measurements have shown that many of the remnants have low-frequency spectral turnovers which are probably due to free-free absorption in localised ionised gas with emission measures > 10⁶ pc cm^–6.