  We have used a Very Long Baseline Interferometry (VLBI) array at 18cm
wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc
linear resolution, and with very high sensitivity. The resulting map has an rms
of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point
sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison
with high sensitivity data from 12 months earlier reveals at least four new
sources. The favored interpretation of these sources is that they are radio
supernovae, and if all new supernovae are detectable at this sensitivity, a
resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per
year. The implied star formation rate is sufficient to power the entire
observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220
exhibit striking similarities in their radio properties, though the western
nucleus is more compact, and appears to be ~3 times more luminous than the
eastern nucleus. There are also some puzzling differences, and differential
free-free absorption, synchrotron aging and expansion losses may all be playing
a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis
that the activity in Arp 220 is essentially a scaled-up version of that in M82.
