NICMOS observations of luminous & ultraluminous infrared galaxies

HST NICMOS observations of a sample of 24 luminous (LIGs: LIR[8-1000 μm] = 1011.0-11.99 L⊙) and ultraluminous (ULIGs:LIR ≳ 1012.0 L⊙) infrared galaxies are presented.The observations provide, for the first time, high resolution HST imagingof the imbedded 1.1 - 2.2 μm nuclear regions of these mergers. Allbut 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 tohave bright nuclei which account for most of the light of the galaxy.This, combined with the tendency for the light of ULIGs to become morecentrally concentrated as a function of increasing wavelength, impliesthat most of their energy is generated within a region 50-200 pcacross.