NICMOS Imaging of the Nuclei of Arp 220

Abstract

We report high-resolution imaging of the ultraluminous infrared galaxyArp 220 at 1.1, 1.6, and 2.22 mu m with the Near-Infrared Cameraand Multiobject Spectrometer on the Hubble Space Telescope. The diffraction-limitedimages at 0.''1-0.''2 resolution clearly resolve both nuclei of themerging galaxy system and reveal for the first time a number of luminousstar clusters in the circumnuclear envelope. The morphologies ofboth nuclei are strongly affected by dust obscuration, even at 2.2mu m: the primary nucleus (west) presents a crescent shape, concaveto the south, and the secondary (eastern) nucleus is bifurcated bya dust lane with the southern component being very reddened. In thewestern nucleus, the morphology of the 2.2 mu m emission is mostlikely the result of obscuration by an opaque disk embedded in thenuclear star cluster. The morphology of the central starburst clusterin the western nucleus is consistent with either a circumnuclearring of star formation or a spherical cluster with the bottom halfobscured by the embedded dust disk. Comparison of centimeter-waveradio continuum maps with the near-infrared images suggests thatthe radio nuclei lie in the dust disk on the west and near the highlyreddened southern component of the eastern complex. The radio nucleiare separated by 0.''98 (corresponding to 364 pc at 77 Mpc), andthe half-widths of the infrared nuclei are ~0.''2-0.''5. At leasteight unresolved infrared sources--probably globular clusters--arealso seen in the circumnuclear envelope at radii of 2''-7''. Theirnear-infrared colors do not significantly constrain their ages.