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The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellentlaboratory for studies of extreme astrophysical environments. For 20 years,Very Long Baseline Interferometry (VLBI) has been used to monitor a populationof compact sources thought to be supernovae (SNe), supernova remnants (SNRs)and possibly active galactic nuclei (AGNs). Using new and archival VLBI dataspanning 20 years, we obtain 23 high-resolution radio images of Arp 220 atwavelengths from 18 cm to 2 cm. From model-fitting to the images we obtainestimates of flux densities and sizes of all detected sources. We detect radiocontinuum emission from 97 compact sources and present flux densities and sizesfor all analysed observation epochs. We find evidence for a LD-relation withinArp 220, with larger sources being less luminous. We find a compact source LF$n(L)\propto L^\beta$ with $\beta=-2.19\pm0.15$, similar to SNRs in normalgalaxies. Based on simulations we argue that there are many relatively largeand weak sources below our detection threshold. The observations can beexplained by a mixed population of SNe and SNRs, where the former expand in adense circumstellar medium (CSM) and the latter interact with the surroundinginterstellar medium (ISM). Nine sources are likely luminous, type IIn SNe. Thisnumber of luminous SNe correspond to few percent of the total number of SNe inArp 220 which is consistent with a total SN-rate of 4 yr$^{-1}$ as inferredfrom the total radio emission given a normal stellar initial mass function(IMF). Based on the fitted luminosity function, we argue that emission from allcompact sources, also below our detection threshold, make up at most 20\% ofthe total radio emission at GHz frequencies.

The population of SNe/SNRs in the starburst galaxy Arp 220. A self-consistent analysis of 20 years of VLBI monitoring