Counterrotating Nuclear Disks in Arp 220

The ultraluminous infrared galaxy Arp 220 has been observed at 0.5"resolution in CO(2-1) and 1 mm continuum using the newly expanded Owens ValleyMillimeter Array. The CO and continuum peaks at the double nuclei and thesurrounding molecular gas disk are clearly resolved. We find steep velocitygradients across each nucleus (dV ~ 500 km/s within r= 0.3") whose directionsare not aligned with each other and with that of the outer gas disk. Weconclude that the double nuclei have their own gas disks (r ~ 100 pc). They arecounterrotating 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 ofeach nucleus are M_dyn > 2* 10^9 M_sun based on the CO kinematics. Althoughthere is no evidence of an old stellar population in the optical or nearinfrared spectroscopy of the nuclei (probably due to the much brighter youngpopulation), it seems likely that these nuclei were 'seeded' from thepre-merger nuclei in view of their counterrotating gas kinematics. The gasdisks probably constitute a significant fraction (~ 50 %) of the mass in eachnucleus. The CO and continuum brightness temperatures imply that the nucleargas disks have high area filling factors (~ 0.5-1) and have extremely highvisual extinctions (Av ~ 1000 mag). The molecular gas must be hot (>= 40 K) anddense (>= 10^4-5 cm^-3), given the large mass and small scale-height of thenuclear disks. The continuum data suggest that the large luminosity (be itstarburst or AGN) must originate within 100 pc of the two nuclear gas diskswhich were presumably formed through concentration of gas from the progenitorouter galaxy disks.Comment: 20 pages, 5 figures. Accepted for publication in The Astrophysical Journa