Ultraluminous infrared galaxies: Atlas of near-infrared images

A sample of 27 ultraluminous infrared galaxy (ULIRG) systems has been imaged at 1.6 mum using the Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer (NICMOS). These ULIRGs are from a larger sample also imaged with HST in the I band. Images and catalog information for the NICMOS subsample, as well as brief morphological descriptions of each system are presented. Inspection of the infrared images and a comparison with optical images of these systems shows that at least 85% are obviously composed of two or more galaxies involved in a close interaction or merger event, with as many as 93% showing some signs of interaction history. Approximately 37% of the systems show either spectroscopic or morphological characteristics of an active galactic nucleus. The infrared morphologies of these systems are generally less complicated or disturbed than their optical morphologies, indicating that some of the small-scale features seen in optical images are likely due to complicated patterns of dust obscuration, as well as widely distributed star formation activity. In some systems the high-resolution HST infrared images have revealed nuclear remnants that are obscured or unidentified in ground-based imaging, which has led to changes in previously determined interaction stage classifications or system content. In general, however, the NICMOS images support previous conclusions from previous HST optical imaging.