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

We report the first submillimeter interferometric observations of anultraluminous infrared galaxy. We observed Arp220 in the CO J=3-2 line and342GHz continuum with the single baseline CSO-JCMT interferometer consisting ofthe Caltech Submillimeter Observatory (CSO) and the James Clerk MaxwellTelescope (JCMT). Models were fit to the measured visibilities to constrain thestructure of the source. The morphologies of the CO J=3-2 line and 342GHzcontinuum emission are similar to those seen in published maps at 230 and110GHz. We clearly detect a binary source separated by about 1 arcsec in theeast-west direction in the 342GHz continuum. The CO J=3-2 visibilityamplitudes, however, indicate a more complicated structure, with evidence for acompact 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 alsoobtained single-dish CO J=2-1, CO J=3-2 and HCN J=4-3 spectra. The HCN J=4-3spectrum, unlike the CO spectra, is dominated by a single redshifted peak. TheHCN 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 largerin the redshifted (eastern) source, which suggests that the two sources mayhave different physical conditions. This result might be explained by thepresence of an intense starburst that has begun to deplete or disperse thedensest gas in the western source, while the eastern source harbors undispersedhigh density gas.Comment: 17 pages, 9 figures, 4 Tables. accepted by Ap