Counterrotating nuclear disks in ARP 220

The ultraluminous infrared galaxy Arp 220 has been observed at 0 ".5 resolu tion in CO (2-1) and 1 mm continuum using the newly expanded Owens Valley M illimeter Array. The CO and continuum peaks at the double nuclei and the su rrounding molecular gas disk are clearly resolved. We find steep velocity g radients across each nucleus (Delta V similar to 500 km s(-1) within I = 0 ".3) whose directions are not aligned with each other and with that of the outer gas disk. We conclude that the double nuclei have their own gas disks (I similar to 100 pc), are counterrotating with respect to each other, and are embedded in the outer gas disk (r similar to 1 kpc) rotating around th e dynamical center of the system. The masses of each nucleus are M-dyn grea ter than or similar to 2 x 10(9) M., based on the CO kinematics. Although t here is no evidence of an old stellar population in the optical or near-inf rared spectroscopy of the nuclei (probably owing to the much brighter young population), it seems likely that these nuclei were "seeded" from the prem erger nuclei in view of their counterrotating gas kinematics. The gas disks probably constitute a significant fraction (similar to 50%) of the mass in each nucleus. The CO and continuum brightness temperatures imply that the nuclear gas disks have high area filling factors (similar to 0.5-1) and ext remely high visual extinctions (A(V) similar or equal to 1000 mag). The mol ecular gas must be hot (greater than or equal to 40 K) and dense (greater t han or equal to 10(4-5) cm(-3)), given the large mass and small scale heigh t of the nuclear disks. The continuum data suggest that the large luminosit y (be it starburst or active galactic nucleus) must originate within less t han or similar to 100 pc of the two nuclear gas disks, which were presumabl y formed through concentration of gas from the progenitor outer galaxy disk s.