Low-frequency observations of supernova remnants in M82

Abstract

We have used MERLIN at 408 MHz to image the central starburst region ofM82 with a resolution of 0.5 arcsec. We detect ~20 supernova remnants at this frequency, which is a much smaller number than we detect at higher frequencies (~50). We attribute this difference to free-free absorption by ionized gas. We find that seven remnants show low-frequency turnovers in their spectra consistent with free-free absorption. These spectra are similarto the spectrum previously obtained for the brightest, youngest supernova remnant, 41.95+57.5. We find a mean optically thin spectral index of -0.6 and emission measures ~ las cm -6 pc for the remnants detected at 408 MHz. These emission measures are an order of magnitude smaller than other published values for the central region of M82 and we therefore suggest that the remnants we detect at 408 MHz are embedded within ionized gas towards the near side of the distribution. In addition to the supernova remnants, we detect the extended background in which several regions attributable to free-free absorption by ionized gas can be seen. The most striking is an extended absorption region (~1 00 pc across) approximately centred on 41.95+57.5 which may be an H II region photoionized by a cluster of early-type stars. The progenitor of 41.95+57.5 may well have been a member of this cluster. We find one source near the dynamical centre of M82, which we speculate could be a possible candidate for an active galactic nucleus.