The double nucleus of Arp 220 unveiled

IR imaging at the f/415 focus of the Palomar 200 inch telescope with 0.39 x 0.49 arcsec resolution is used to determine that the ultraluminous IRAS galaxy Arp 220 has a double nucleus with separation 0.95 arcsec. The presence of two closely separated nuclei (330 pc) confirms the circumstantial evidence that Arp 220 is an evolved merger remnant; and the degree of correlation between the IR and cm wavelength radio emission shows that it is an ongoing merger containing two active nuclei accompanied by circumnuclear starbursts of moderate intensity. The extent of the 2.2 micron radiation is evidence that the flux is most probably dominated by starlight. It is pointed out that 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. 22 refs