The ultraluminous infrared galaxy Arp 220 has been observed at 0 \farcs 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 ( \Delta V \sim 500 km s ^ { -1 } within r = 0 \farcs 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 \sim 100 pc ) .
They are counterrotating with respect to each other and embedded in the outer gas disk ( r \sim 1 kpc ) rotating around the dynamical center of the system .
The masses of each nucleus are M _ { dyn } \gtrsim 2 \times 10 ^ { 9 } \mbox { $M _ { \odot } $ } 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 were ’ seeded ’ from the pre-merger nuclei in view of their counterrotating gas kinematics .
The gas disks probably constitute a significant fraction ( \sim 50 % ) of the mass in each nucleus .
The CO and continuum brightness temperatures imply that the nuclear gas disks have high area filling factors ( \sim 0.5–1 ) and have extremely high visual extinctions ( A _ { V } \simeq 1000 mag ) .
The molecular gas must be hot ( \geq 40 K ) and dense ( \geq 10 ^ { 4 - 5 } cm ^ { -3 } ) , given the large mass and small scale-height of the nuclear disks .
The continuum data suggest that the large luminosity ( be it starburst or AGN ) must originate within \lesssim 100 pc of the two nuclear gas disks which were presumably formed through concentration of gas from the progenitor outer galaxy disks .