LOW-FREQUENCY OBSERVATIONS OF SUPERNOVA-REMNANTS IN MS2

We have used MERLIN at 408 MHz to image the central starburst region o f M82 with a resolution of 0.5 arcsec. We detect similar to 20 superno va remnants at this frequency, which is a much smaller number than we detect at higher frequencies (similar to 50). We attribute this differ ence to free-free absorption by ionized gas. We find that seven remnan ts show low-frequency turnovers in their spectra consistent with free- free absorption. These spectra are similar to 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 measur es similar to 10(5) cm(-6) pc for the remnants detected at 408 MHz. Th ese emission measures are an order of magnitude smaller than other pub lished values for the central region of M82 and we therefore suggest t hat the remnants we detect at 408 MHz are embedded within ionized gas towards the near side of the distribution. In addition to the supernov a 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 (similar to 100 pc acro ss) approximately centred on 41.95+57.5 which may be an H II region ph otoionized by a cluster of early-type stars. The progenitor of 41.95+5 7.5 may well have been a member of this cluster. We find one source ne ar the dynamical centre of M82, which we speculate could be a possible candidate for an active galactic nucleus.