The visible image of Arp220 was just interesting enough to warrant its inclusion in Halton Arp's 1966 Atlas of Peculiar Galaxies. But only now are we beginning to see how very peculiar this galaxy really is. While it ranks behind about 10 000 other known galaxies in visible brightness, the recent Infrared Astronomy Satellite (IRAS) survey has shown Arp220 to be one of the ten brightest extragalactic infrared sources in the sky. Almost 99% of its total energy output is in the infrared. From the red shift of its visible line spectrum one deduces a distance of about 100 Mpc (300 million light years). At this distance its observed infrared brightness implies a total luminosity of 2×1012 times that of the Sun. This astonishing luminosity, a hundred times that of normal galaxies like our own, is exceeded only by the most luminous quasars. Quasars, however, emit strongly at visible and shorter wavelengths.

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