Counterrotating Nuclear Disks in Arp 220

Abstract

The ultraluminous infrared galaxy Arp 220 has been observed at 0.5''resolution in CO (2-1) and 1 mm continuum using the newly expandedOwens Valley Millimeter Array. The CO and continuum peaks at thedouble nuclei and the surrounding molecular gas disk are clearlyresolved. We find steep velocity gradients across each nucleus (DeltaV~500km s^-1 within r=0.3'') whose directions are not aligned witheach other and with that of the outer gas disk. We conclude thatthe double nuclei have their own gas disks (r~100 pc), are counterrotatingwith respect to each other, and are 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{\gt}~2x10^9 M_solar,based on the CO kinematics. Although there is no evidence of an oldstellar population in the optical or near-infrared spectroscopy ofthe nuclei (probably owing to the much brighter young population),it seems likely that these nuclei were ``seeded'' from the premergernuclei in view of their counterrotating gas kinematics. The gas disksprobably constitute a significant fraction (~50%) of the massin each nucleus. The CO and continuum brightness temperatures implythat the nuclear gas disks have high area filling factors (~0.5-1)and extremely high visual extinctions (A_V~=1000 mag). The moleculargas must be hot ({\gt}=40 K) and dense ({\gt}=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 starburstor active galactic nucleus) must originate within {\lt}~100 pcof the two nuclear gas disks, which were presumably formed throughconcentration of gas from the progenitor outer galaxy disks.