ABSTRACT. We have used a very long baseline interferometry (VLBI) array at 18 cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, with very high sensitivity. The resulting map has an rms of 5.5 μJy beam^-1, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 μJy. 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, although 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.

Subject headings: galaxies: active; galaxies: individual (Arp 220); galaxies: starburst; infrared: galaxies; radio continuum: galaxies; supernovae: general

Print publication: Issue 1 (2006 August 10)
Received 2006 March 6, accepted for publication 2006 April 14