Dust and gas in the core of Arp 220 (IC 4553)

New higher sensitivity and higher angular resolution data are reported for the CO emission and adjacent 2.7 mm continuum in the ultraluminous IR galaxy Arp 220 are reported. Two emission components are identified, an extended component containing one-third of the flux and a compact, nuclear component containing two-thirds of the flux. The extended component has a size of 2.5 x 6.5 kpc; the major axis ia approximately aligned with the optical dust lane. The nuclear source has a radius of 315 pc. Velocity gradients with the same sense of rotation are seen in both components. The mass of gas in the central concentration is estimated at 25 billion solar masses. A correlation is found between the total luminosity-to-gas mass ratio and the central gas surface density. A central dust-embedded active nucleus is the favored mechanism for luminosity generation in Arp 220. 30 refs.