High Resolution Tip/Tilt Near-Infrared Imaging of Warm Ultraluminous   Infrared Galaxies
  We present high spatial resolution H and K'-band images of a complete sample of ultraluminous infrared galaxies selected to have warm mid-infrared colors known to be characteristic of active galaxies. The extended underlying galaxy is detected in each system, as are tidal features and many of the star-forming knots seen at optical wavelengths. While some of these knots have a considerable near-infrared excess, we show that they are likely to have bolometric luminosities more similar to that of the extended starbursts with similar optical morphology seen in less far-infrared luminous interacting systems. We find that each ULIG has increasing contributions at long wavelengths by a very compact source which we identify as an active galactic nucleus. We show that the optical/near-infrared colors of these putative nuclei are much more extreme than the most active starburst IRAS galaxies, yet are identical to "far-infrared loud" quasars which are in turn similar to optical quasars combined with large quantities of hot dust. Half of the ULIGs have nuclei with dereddened near-infrared luminosities comparable to those of QSOs, while the other half have dereddened luminosities more similar to Seyferts, although this may be an effect of patchy extinction and scattering. 
Jason A. Surace, D.B. Sanders