  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 ~100 pc in diameter, while five are in the
southwestern nucleus (component "B1"=NGC 3690) within a region ~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 Cas 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 2,000 times the present power of
Cassiopeia 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.
