NICMOS observations of luminous & ultraluminous infrared galaxies

HST NICMOS observations of a sample of 24 luminous (LIGs: L-IR [8-1000 mu m] = 10(11.0-11.99) L.) and ultraluminous (ULIGs: L-IR greater than or similar to 10(12.0) L.) infrared galaxies are presented. The observations provide, for the first time, high resolution HST imaging of the imbedded 1.1 - 2.2 mu m nuclear regions of these mergers. All but one of the ULIGs are observed to have at least one compact (50-200 pc) nucleus, and more than half contain what appear to be blue star clusters. The warm infrared galaxies (i.e., the transition sources) are observed to have bright nuclei which account for most of the light of the galaxy. This, combined with the tendency for the light of ULIGs to become more centrally concentrated as a function of increasing wavelength, implies that most of their energy is generated within a region 50-200 pc across.