Two radio supernovae in the unusual galaxy Markarian 297

Abstract

STARBURST galaxies have higher star formation and supernova rates than normal galaxies, triggered in many cases by the interaction of two galaxies¹. Direct estimates of the supernova rate in starburst galaxies is difficult, because optical direction of super-novae is hampered by high background brightness and by dust², both of which are associated with the starburst phenomenon. Radio observations do not suffer from these problems. We report here the discovery of two radio supernovae in the clumpy irregular galaxy Markarian 297 (Mkn 297, also known as NGC6052), which has been estimated³ to have a star formation rate ten times that of a normal spiral galaxy, and a supernova rate of 0.3–0.5 per year. It may consist of two interacting galaxies⁴. A variable radio source found in 1982 (ref. 5) was attributed to possible supernova activity, and our observations with the Very Large Array, combined with earlier published data, define a reasonable radio light curve for a supernova that exploded in July 1979. The variability resembles that of the young supernova SN1986J (ref. 6), but at maximum the supernova in Mkn 297 would have been about six times as bright. We also identify a second source in the galaxy as a supernova. The presence of two extraordinarily bright supernovae in the same region may be a particular result of the interaction between two galaxies.