ARCSECOND IMAGING OF CO EMISSION IN THE NUCLEUS OF ARP-220

We report high-resolution (1 '') imaging of CO (2-1) and dust continuu m emission in the ultraluminous galaxy Arp 220. The CO (1-0) line was also imaged at 2 '' resolution for comparison. Both data sets recover essentially all of the observed single-dish line emission. Our apertur e synthesis maps reveal for the first time, multiple components in the dense gas: peaks corresponding to each of the double nuclei (separate d by 0.95 '' at P.A. = 101 degrees) seen in the near infrared and radi o continuum and a more extended disklike structure at P.A. = 53 degree s, similar to the dust lane seen in optical images. Approximately two- thirds of the total CO emission (and presumably the H-2 mass) coincide s with the compact double nucleus region. The ISM associated with thes e nuclear sources is most apparent in the 1.3 mm dust continuum emissi on, but the brightest CO (2-1) emission is also correlated with the ne ar infrared nuclei and exhibits a radial velocity difference of 250-30 0 km s(-1) between the two nuclei. The latter is in excellent agreemen t with published near-infrared recombination line measurements. The ob served velocity difference between the two nuclei is probably much les s than their orbital velocity because the nuclei do not lie along the kinematic major axis of the inner disk. The elongated disk feature exh ibits a monotonic velocity gradient parallel to the major axis of the CO intensity distribution with the highest receding velocities in the southwest and the highest approach velocities in the northeast. From t he major/minor axis ratio (0.66), we infer that the disk is moderately inclined to the line of sight (i = 40-50 degrees). Detailed modeling of the CO line profiles using a Doppler image-deconvolution technique, analogous to Doppler radar imaging, yields a best-fit CO emissivity d istribution and rotation curve which are mutually consistent in the se nse that if the total mass distribution follows the CO emissivity, the n it yields the derived rotation curve. The implied CO-to-H-2 conversi on ratio is 0.45 times the Galactic value if the bulk of the mass resi des in the molecular gas, rather than stars. This value is also consis tent with that expected based on the likely molecular density and temp erature in the nuclear disk of Arp 220. The total molecular gas mass f or Arp 220 is similar to 9 x 10(9) M. with an uncertainty of similar t o 30% based on the line profile modeling. The peak gas surface density is similar to 5.8 x 10(4) M. pc(-2) at 130 pc radius, while the two s tellar nuclei are at similar to 235 pc radius and at position angle mi dway between the major and minor axes of the gaseous disk. From the pr ofile modeling we derive an intrinsic velocity dispersion in the disk of 90 +/- 20 km s(-1) and thus a disk thickness (FWHM) of only 16 pc, assuming the disk is in hydrostatic equilibrium. With 5.4 x 10(9) M. o f molecular gas concentrated in the very thin disk associated with the twin nuclei, the mean density will be n(H2) similar or equal to 2 x 1 0(4) cm(-3) (+/-30%), a value which is consistent with the strong mole cular emission from high dipole moment molecules such as HCN and HCO+. From the high brightness temperatures of the observed CO emission (17 -21 K), we conclude that the area filling factor of the disk is very h igh (similar or equal to 0.25) and therefore the gas must fairly unifo rmly fill the disk, rather than being in discrete self-gravitating clo uds. This thin central disk will have inward accretion at similar or e qual to 100 M. yr(-1) due to viscous and spiral arm transfer of angula r momentum. The line profiles at the positions of the double nuclei ar e double peaked suggesting that there may also be less massive accreti on disks associated with each nucleus. The fact that the bulk of the m olecular gas has relaxed into a disk with large masses of gas concentr ated interior to the double nuclei is consistent with scenarios in whi ch the gas in merging systems settles into the center faster than the two stellar/starburst nuclei. We suggest that dense central accretion disks like that in Arp 220 may be a common feature in the evolution of ultraluminous starburst/AGN galaxies since similar qualitative featur es are seen in the molecular line data for other systems (e.g., Mrk 23 1 and NGC 6240).