2000ApJ...529..170S

We present high spatial resolution (FWHM~0".3-0".8) BIHK'-band imaging of a sample of ultraluminous infrared galaxies (L_ir>10¹² L_☉; ULIGs) with ``cool'' mid-infrared colors (f<sub>25µm</sub>/f<sub>60µm</sub><0.2), which select against active galactic nucleus-like (AGN-like) systems, a complementary sample to the ``warm'' ULIGs of Surace and coworkers. We find that all of the cool ULIGs are either advanced mergers or are premergers with evidence for still-separate nuclei with separations greater than 600 pc. Extended tidal features such as tails and loops as well as clustered star formation are observed in most systems. This extended tidal structure suggests a common progenitor geometry for most of the ULIGs: a plunging disk collision where the disks are highly inclined with respect to each other. The underlying host galaxies have H-band luminosities of 1-2.5L^*, very similar to that found in the warm ULIGs. The nuclear regions of these galaxies have morphologies and colors characteristic of a recent burst of star formation mixed with hot dust and mildly extinguished by A_V=2-5 mag; only in one case (IRAS 22491-1808) is there evidence for a compact emission region with colors similar to those ofan extinguished QSO. Most of the observed star-forming knots appear to have very young (10 Myr) ages based on their optical/near-infrared colors. These star-forming knots are insufficiently luminous to typically provide more than 10% of the high bolometric luminosity of the systems.