K. Sakamoto

The ultraluminous infrared galaxy Arp 220 has been observed at 0: 00 5 resolution in CO(2--1) and 1 mm continuum using the newly expanded Owens Valley Millimeter Array. The CO and continuum peaks at the double nuclei and the surrounding molecular gas disk are clearly resolved. We find steep velocity gradients across each nucleus (\DeltaV 500 km s \Gamma1 within r = 0: 00 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 (r 100 pc). They are counterrotating with respect to each other and embedded in the outer gas disk (r 1 kpc) rotating around the dynamical center of the system. The masses of each nucleus are M dyn ? 2 \Theta 10 9 M fi based on the CO kinematics. Although there is no evidence of an old stellar population in the optical or near infrared spectroscopy of the nuclei (probably due to the much brighter young population), it seems likely that these nuclei w..