The supernova rate in local galaxy clusters

We report a measurement of the supernova (SN) rates (Type la and core-collapse) in galaxy clusters based on the 136 supernovae of the sample described in Cappellaro, Evans & Turatto, and Mannucci et al. Early-type cluster galaxies show a Type la SN rate (0.066 SNuM) similar to that obtained by Sharon et al. and more than three times larger than that in field early-type galaxies (0.019 SNuM). This difference has a 98 percent statistical confidence level. We examine many possible observational biases which could affect the rate determination, and conclude that none of them is likely to significantly alter the results. We investigate how the rate is related to several properties of the parent galaxies, and find that cluster membership, morphology and radio power all affect the SN rate, while galaxy mass has no measurable effect. The increased rate may be due to galaxy interactions in clusters, inducing either the formation of young stars or a different evolution of the progenitor binary systems. We present the first measurement of the core-collapse SN rate in cluster late-type galaxies, which turns out to be comparable to the rate in field galaxies. This suggests that no large systematic difference in the initial mass function exists between the two environments.