NICMOS Imaging of the Nuclei of Arp 220

Abstract

We report high-resolution imaging of the ultraluminous infrared galaxy Arp 220 at 1.1, 1.6, and 2.22 μm with the Near-Infrared Camera and Multiobject Spectrometer on the Hubble Space Telescope. The diffraction-limited images at 01-02 resolution clearly resolve both nuclei of the merging galaxy system and reveal for the first time a number of luminous star clusters in the circumnuclear envelope. The morphologies of both nuclei are strongly affected by dust obscuration, even at 2.2 μm: the primary nucleus (west) presents a crescent shape, concave to the south, and the secondary (eastern) nucleus is bifurcated by a dust lane with the southern component being very reddened. In the western nucleus, the morphology of the 2.2 μm emission is most likely the result of obscuration by an opaque disk embedded in the nuclear star cluster. The morphology of the central starburst cluster in the western nucleus is consistent with either a circumnuclear ring of star formation or a spherical cluster with the bottom half obscured by the embedded dust disk. Comparison of centimeter-wave radio continuum maps with the near-infrared images suggests that the radio nuclei lie in the dust disk on the west and near the highly reddened southern component of the eastern complex. The radio nuclei are separated by 098 (corresponding to 364 pc at 77 Mpc), and the half-widths of the infrared nuclei are ~02-05. At least eight unresolved infrared sources—probably globular clusters—are also seen in the circumnuclear envelope at radii of 2''-7''. Their near-infrared colors do not significantly constrain their ages.