The double nucleus of ARP 220 unveiled
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Abstract
Infrared imaging at the f/415 focus of the Palomar 200 inch (5 m) telescope with 0.39" x 0.49" resolution is used to show that the ultraluminous IRAS galaxy Arp 220 has a double nucleus with separation 0.95". This high resolution was achieved by taking a series of 5 s exposures, centroiding, shifting to a common origin, and then co-adding. The presence of two closely separated nuclei (330 pc) in Arp 220 confirms the circumstantial evidence--disturbed optical morphology, remnants of tidal tails, and nonconcentric infrared isophotes--that it is an evolved merger remnant. Both nuclei are clearly resolved showing barlike morphology. Tbe extent of the 2.2 micron radiation is evidence that the flux is most probably dominated by starlight. A remarkable degree of correlation is noted between the infrared and centimeter wavelength radio emission and is used to show that Arp 220 is an ongoing merger containing two active nuclei accompanied by circumnuciear starbursts of moderate intensity. If the nuclear activity is powered by accretion onto black holes, then a black hole binary will be formed. Such a binary may be an essential ingredient of many quasars, and therefore Arp 220 may suggest another connection between mergers, ultraluminous IRAS galaxies, and quasars.
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