RADIO EMISSION FROM YOUNG SUPERNOVAE AND SUPERNOVA REMNANTS IN Arp 299

We have made sensitive milliarcsecond-resolution radio images of the nearby merger galaxy Arp 299 at four epochs spread over 18 months between 2003 and 2005. The combined data revealed a total of 30 point sources in the two primary merger nuclei. Twenty-five of these are found in the northeastern nucleus (component A = IC 694) over a region similar to 100 pc in diameter, while five are in the southwestern nucleus (component B1 = NGC 3690) within a region similar to 30 pc in size. These objects are interpreted as young supernovae and supernova remnants; the ratio of the source counts in nuclei A and B1 is approximately equal to the ratio of their predicted supernova rates. An approximate luminosity function has been derived for nucleus A, and indicates that it might contain as many as 500-1000 compact radio sources more powerful than Cassiopeia A; the integrated flux density of these sources would be about 20% of the total flux density seen at lower resolution. A new supernova occurred in nucleus B1 in the first half of 2005, having a peak radio power at least 2000 times the present power of Cas A. This supernova is located within 0.4 pc (projected distance) of an apparently older supernova remnant, making it very likely that this indicates the presence of a massive super star cluster within nucleus B1. Comparison of the typical radio flux densities of our compact radio sources to the observed X-ray luminosities of nuclei A and B1 indicates that it is possible that one radio source in each nucleus actually could be associated with an active galactic nucleus rather than being a supernova remnant.