High-resolution TIP/TILT near-infrared imaging of warm ultraluminous infrared galaxies

We present high spatial resolution (FWHM approximate to 0 ".-0 ".5) H (1.6 mu m) and K' (2.1 mu m) images of a complete sample (12) of ultraluminous infrared galaxies (ULIGs) chosen to have "warm" mid-infrared colors (f(25)/f(60) > 0.2) known to be characteristic of active galaxies. The extended underlying galaxy is detected in each system at H and K', as are tidal features and many of the star-forming knots seen at optical wavelengths. While some of these knots have considerable near-infrared excesses, we show that they are likely to have bolometric luminosities more similar to those 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 (AGN). We show that the optical/near-infrared colors of these putative nuclei are much more extreme than in the most active starburst IRAS galaxies, yet are identical to those in "far-infrared-loud" quasars, which are in turn similar to optical quasars combined with large quantities of hot (approximate to 800 K) 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 Seyfert galaxies, although this may be an effect of patchy extinction and scattering.