VLBI Images of 49 Radio Supernovae in Arp 220

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