Why are radio galaxies prolific producers of type Ia supernovae?

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(-0.14)(+0.19) h(75)(2) SNu as compared to SN Ia rate(radio-quiet galaxies) 0.11(-0.03)(+0.06) h(2)(75) SNu. The actual value of the enhancement is likely to be in the range similar to 2-7 (P similar to 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 similar to 14% of early-type galaxies and supplying an adequate number of SN Ia progenitors to the stellar population of elliptical galaxies.