Why Are Radio Galaxies Prolific Producers of Type Ia Supernovae?

Abstract

An analysis of SN Ia events in early-type galaxies from the database of Cappellaro and coworkers provides conclusive evidence that the rate of SNe Ia in radio-loud galaxies is about 4 times higher than the rate measured in radio-quiet galaxies, i.e., SN Ia rate(radio-loud galaxies) = 0.43 h SNu as compared to SN Ia rate(radio-quiet galaxies) = 0.11 h SNu. The actual value of the enhancement is likely to be in the range ~2-7 (P ~ 10-4). This finding puts on robust empirical grounds the results obtained by Della Valle & Panagia on the basis of a smaller sample of SNe. We analyze the possible causes of this result and conclude that the enhancement of the SN Ia explosion rate in radio-loud galaxies has the same origin as their being strong radio sources, but there is no causal link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early-type galaxies with dwarf companions, on timescales of about 1 Gyr, are responsible for both inducing strong radio activity observed in ~14% of early-type galaxies and supplying an adequate number of SN Ia progenitors to the stellar population of elliptical galaxies.