Evidence for an Expanding Molecular Superbubble in M82

Abstract

We present evidence for an expanding superbubble in M82 (diameter: ~130 pc, expansion velocity: ~45 km/s, mass: ~8*10^6 SM). It is seen in the 12CO(J=1-0), 12CO(J=2-1), 13CO(J=1-0) and C18O(J=1-0) lines. The superbubble is centred around the most powerful supernova remnant in M82, 41.9+58. The CO observations show that the molecular superbubble already broke out of the disk. This scenario is supported by ROSAT HRI observations which suggest that hot coronal gas originating from inside the shell is the main contributor to the diffuse X-ray outflow in M82. We briefly discuss observations of the same region at other wavelengths (radio continuum, optical, HI, X-rays, ionized gas). From our spectral line observations, we derive a kinematic age of about 10^6 years for the superbubble. Using simple theoretical models, the total energy needed for the creation of this superbubble is of order 2*10^{54} ergs. The required energy input rate (0.001 SN yr^{-1}) is reasonable given the high supernova (SN) rate of ~0.1 SN yr^{-1} in the central part of M82. As much as 10% of the energy needed to create the superbubble is still present in form of the kinetic energy of the expanding molecular shell. Of order 10% is conserved in the hot X-ray emitting gas emerging from the superbubble into the halo of M82. This newly detected expanding molecular superbubble is believed to be powered by the same objects that also lie at the origin of the prominent X-ray outflow in M82. It can therefore be used as an alternative tool to investigate the physical properties of these sources.