The radio spectra of the compact sources in Arp 220: A mixed population of supernovae and supernova remnants

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of the compact sources within both nuclei of the ultraluminous infrared galaxy Arp 220, presenting radio spectra of the 18 detected sources. In just over half of these, we find that these spectra and other properties are consistent with the standard model of powerful Type IIn supernovae interacting with their preexplosion stellar wind. The rate of appearance of new radio sources identified with these supernova events suggests that an unusually large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function or stellar evolution compared to galactic disks. Another possible explanation invokes very short (similar to 3; 10(5) yr) intense (similar to 10(3) M-circle dot yr(-1)) star formation episodes with a duty cycle of similar to 10%. A second group of our detected sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. These sources show a range of spectral indexes from -0.2 to -1.9. We propose that these are young supernova remnants that have just begun interacting with a surrounding ISM with a density between 10(4) and 10(5) cm(-3). One source is probably resolved at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate that the ionized component of the ISM gives rise to foreground free-free absorption with opacity at 18 cm of < 0.6 along the majority of lines of sight. Other sources may be affected by absorption with opacity in the range 1-2. These values are consistent with previous models as fitted to the radio recombination lines and the continuum spectrum.