A Supernova Factory in the Merger System Arp 299

We have imaged the nearby galaxy merger Arp 299 at arcsecond andmilliarcsecond resolution, using both the Very Large Array and the Very LongBaseline Array. The large-scale radio emission from the merger contains 5bright, compact radio sources embedded in diffuse emission, with diameters lessthan 200 pc. Supernova rates of 0.1 to 1 per year are required to produce theVLA-detected radio emission in these sources. Two of the compact VLA radiosources, designated Source A and Source D, also have been detected and imagedat milliarcsecond scales. Source A, which is associated with the nucleus of oneof the merging galaxies, contains five milliarcsecond-scale sources, each witha radio power between 100 and 1000 times that of the Galactic supernova remnantCassiopeia A. Four of these have flat or inverted spectra and appear to beyoung 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, indicatingthat they all may be within the same super starcluster or complex of suchclusters. The brightest VLBI-scale source, A0, has an extremely invertedpectrum, with alpha larger than +2 at gigahertz frequencies. It seems to be theyoungest supernova, which has not yet broken out of its circumstellar shell.The milliarcsecond radio sources within Source A appear to constitute aupernova factory, confirming the presence of an extreme starburst that peakedat least a few million years ago.Comment: Accepted for the Astrophysical Journal, 22 pages, 10 figure