Interferometric Observations of the Nuclear Region of Arp 220 at Submillimeter Wavelengths

We report the first submillimeter interferometric observations of an ultraluminous infrared galaxy. We observed Arp 220 in the CO J = 3-2 line and 342 GHz continuum with the single-baseline CSO-JCMT interferometer consisting of the Caltech Submillimeter Observatory (CSO) and the James Clerk Maxwell Telescope (JCMT). Models were fitted to the measured visibilities to constrain the structure of the source. The morphologies of the CO J = 3-2 line and 342 GHz continuum emission are similar to those seen in published maps at 230 and 110 GHz. We clearly detect a binary source separated by ~1'' in the east-west direction in the 342 GHz continuum. The CO J = 3-2 visibility amplitudes, however, indicate a more complicated structure, with evidence for a compact binary at some velocities and rather more extended structure at others. Less than 30% of the total CO J = 3-2 emission is detected by the interferometer, which implies the presence of significant quantities of extended gas. We also obtained single-dish CO J = 2-1, CO J = 3-2, and HCN J = 4-3 spectra. The HCN J = 4-3 spectrum, unlike the CO spectra, is dominated by a single redshifted peak. The HCN J = 4-3/CO J = 3-2, HCN J = 4-3/HCN J = 1-0, and CO J = 3-2/2-1 line ratios are larger in the redshifted (eastern) source, which suggests that the two sources may have different physical conditions. This result might be explained by the presence of an intense starburst that has begun to deplete or disperse the densest gas in the western source, while the eastern source harbors undispersed high-density gas.