Arcsecond Imaging of CO Emission in the Nucleus of Arp 220

Abstract

We report high-resolution (1'') imaging of CO (2--1) and dust continuumemission in the ultraluminous galaxy Arp 220. The CO (1--0) linewas also imaged at 2'' resolution for comparison. Both data setsrecover essentially all of the observed single-dish line emission.Our aperture synthesis maps reveal for the first time, multiple componentsin the dense gas: peaks corresponding to each of the double nuclei(separated by 0.''95 at P.A. = 101 deg) seen in the near infraredand radio continuum and a more extended disklike structure at P.A.= 53 deg, similar to the dust lane seen in optical images. Approximatelytwo-thirds of the total CO emission (and presumably the H2 mass)coincides with the compact double nucleus region. The ISM associatedwith these nuclear sources is most apparent in the 1.3 mm dust continuumemission, but the brightest CO (2--1) emission is also correlatedwith the near infrared nuclei and exhibits a radial velocity differenceof 250--300 km s-1 between the two nuclei. The latter is in excellentagreement with published near-infrared recombination line measurements.The observed velocity difference between the two nuclei is probablymuch less than their orbital velocity because the nuclei do not liealong the kinematic major axis of the inner disk. The elongated diskfeature exhibits a monotonic velocity gradient parallel to the majoraxis of the CO intensity distribution with the highest receding velocitiesin the southwest and the highest approach velocities in the northeast.From the major/minor axis ratio (0.66), we infer that the disk ismoderately inclined to the line of sight (i = 40--50 deg). Detailedmodeling of the CO line profiles using a Doppler image-deconvolutiontechnique, analogous to Doppler radar imaging, yields a best-fitCO emissivity distribution and rotation curve which are mutuallyconsistent in the sense that if the total mass distribution followsthe CO emissivity, then it yields the derived rotation curve. Theimplied CO-to-H2 conversion ratio is 0.45 times the Galactic valueif the bulk of the mass resides in the molecular gas, rather thanstars. This value is also consistent with that expected based onthe likely molecular density and temperature in the nuclear diskof Arp 220. The total molecular gas mass for Arp 220 is ~9 x 109M{\.o} with an uncertainty of ~30% based on the line profilemodeling. The peak gas surface density is ~5.8 x 104 M{\.o} pc-2at 130 pc radius, while the two stellar nuclei are at ~235 pcradius and at position angle midway between the major and minor axesof the gaseous disk. From the profile modeling we derive an intrinsicvelocity dispersion in the disk of 90 +/- 20 km s-1 and thus a diskthickness (FWHM) of only 16 pc, assuming the disk is in hydrostaticequilibrium. With 5.4 x 109 M{\.o} of molecular gas concentratedin the very thin disk associated with the twin nuclei, the mean densitywill be n_{{H}_{2}} ~= 2 x 104 cm-3 (+/-30%), avalue which is consistent with the strong molecular emission fromhigh dipole moment molecules such as HCN and HCO+. From the highbrightness temperatures of the observed CO emission (17--21 K), weconclude that the area filling factor of the disk is very high (~=0.25)and therefore the gas must fairly uniformly fill the disk, ratherthan being in discrete self-gravitating clouds. This thin centraldisk will have inward accretion at ~=100 M{\.o} yr-1 due to viscousand spiral arm transfer of angular momentum. The line profiles atthe positions of the double nuclei are double peaked suggesting thatthere may also be less massive accretion disks associated with eachnucleus. The fact that the bulk of the molecular gas has relaxedinto a disk with large masses of gas concentrated interior to thedouble nuclei is consistent with scenarios in which the gas in mergingsystems settles into the center faster than the two stellar/starburstnuclei. We suggest that dense central accretion disks like that inArp 220 may be a common feature in the evolution of ultraluminousstarburst/AGN galaxies since similar qualitative features are seenin the molecular line data for other systems (e.g., Mrk 231 and NGC6240).