Radio Observations of Supernova Remnants in the M82 Starburst

We report on recent MERLIN, VLA and VLBI observations of the compact radio sources in the nearby starburst M82, with angular resolutions ranging from arcseconds to milliarcseconds. The spectral properties of the compact sources have been investigated by 15 GHz VLA-Pie Town observations which show that 16 of the less luminous compact sources are, in fact, HII regions. However the steep non-thermal spectrum, parsec size and small variability of the remaining sources is consistent with their being supernova remnants. Several show clear shell structures at MERLIN resolution (˜50 mas) and 5 have been resolved further using VLBI. Measurements of the most compact source (41.95+575) shows an expansion velocity of ˜ 2000 km s^-1, and one of the best defined SNR shells (43.31+592) shows an expansion velocity of ˜ 10⁴ km s^-1. Recent VLBI and MERLIN measurements confirm this expansion velocity and show little evidence for deceleration. We comment on the discrepancy between this measured expansion velocity and the low expansion velocities predicted theoretically for remnants in M82.