High-resolution Optical/near-infrared Imaging of Cool Ultraluminous Infrared Galaxies

We present high spatial resolution BIHK@-band imaging of a sample of ultralu(FWHMB 0A.3È0A.8) minous infrared galaxies ULIGs) with ““ cool ÏÏ mid-infrared colors (L ir[ 1012 L _ ; ( f25 km/f60 km\ 0.2), which select against active galactic nucleusÈlike (AGN-like) systems, a complementary sample to the ““ warm ÏÏ ULIGs of Surace and coworkers. We Ðnd 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 1È2.5L *, 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 mag ; only in one case (IRAS A V \ 2È5 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. Subject headings : galaxies : active È galaxies : interactions È galaxies : star clusters È galaxies : starburst È infrared : galaxies