A supernova factory in the merger system ARP 299

We have imaged the nearby galaxy merger Arp 299 at arcsecond and milliarcsecond resolution, using both the Very Large Array (VLA) and the Very Long Baseline Array. The large-scale radio emission from the merger contains five bright, compact radio sources embedded in diffuse emission, with diameters less than 200 pc. Supernova rates of 0.1-1 yr(-1) are required to produce the VLA-detected radio emission in these sources. Two of the compact VLA radio sources, designated source A and source D, have also been detected and imaged at milliarcsecond scales. Source A, which is associated with the nucleus of one of the merging galaxies, contains five milliarcsecond-scale sources, each with a radio power between 100 and 1000 times that of the Galactic supernova remnant Cassiopeia A. Four of these have flat or inverted spectra and appear to be young supernovae. Three of the VLBI-scale sources are located within 10 pc (projected) of one another, and two are separated by less than 3 pc, indicating that they all may be within the same super-star cluster or complex of such clusters. The brightest VLBI-scale source, A0, has an extremely inverted spectrum, with alphagreater than or similar to+2 at gigahertz frequencies. It seems to be the youngest supernova, having not yet broken out of its circumstellar shell. The milliarcsecond radio sources within source A appear to constitute a supernova factory, confirming the presence of an extreme starburst that peaked at least a few million years ago.