Starburst at the Molecular Superbubble in M82

Abstract

We present high spatial resolution 100 GHz continuum emission observations with the Nobeyama Millimeter Array (NMA) toward an expanding molecular superbubble in the central region of M82. The 100 GHz continuum image, which is dominated by free-free emission, revealed that the strongest peaks are concentrated at the inner edge of the superbubble along the galactic disk. Star formation at these peaks are an order of magnitude more active in terms of ionizing flux than those from the most massive star forming regions in our Galaxy. At this region, high velocity ionized gas, masers, and diffuse hard X-ray emission are also concentrated. These observations suggest that past strong starbursts produced plasma and the superbubble expansion, and induced the present starburst regions. These results provide the first clear evidence of self-propagating starburst in external galaxies.