2007A&A...468L..57D

We present new observations with the IRAM Interferometer, in its longest-baseline configuration, of the CO(2-1) line and the 1.3mm dust radiation from the Arp 220 nuclear region. The dust source in the West nucleus has a size of 0.19''x0.13'' and a 1.3mm brightness temperature of 90K. This implies that the dust ring in the West nucleus has a high opacity, with τ=1 at 1.1mm. Not only is the dust ring itself optically thick in the submm and far-IR, but it is surrounded by the previously-known, rapidly rotating molecular disk of size 0.5'' that is also optically thick in the mid-IR. The molecular ring is cooler than the hot dust disk because the CO(2-1) line is seen in absorption against the dust disk. The dust ring is massive (10⁹M_☉), compact (radius 35pc), and hot (true dust temperature 170K). It resembles rather strikingly the dust ring detected around the quasar APM 08279+52, and is most unlike the warm, extended dust sources in starburst galaxies. Because there is a strong temperature gradient from the hot dust ring to the cooler molecular disk, the heating must come from a concentrated source, an AGN accretion disk that is completely invisible at optical wavelengths, and heavily obscured in hard X-rays.