Nature of widely separated ultraluminous infrared galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by D. C. Kim, about 5% consists of widely separated galaxies which are pre sumably in the early phase of interaction. This fact is contrary to the con ventional view that ULIRGs are in the final stages of the merger of two gas -rich disk galaxies. We have undertaken high-resolution CO (J = 1-0) observ ations for the ultraluminous infrared galaxies that have nuclear separation s larger than 20 kpc. We have detected CO emission in five out of six syste ms, but only in one component of the ULIRG pairs. Four of them have LINER s pectral type and one galaxy has Seyfert 2 spectral type. In K'-band images these components are also brighter than the other components which have eit her H II region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few time s 10(10) similar to that of the other ultraluminous galaxies. The result in dicates that the galaxy containing M-., the molecular gas is also the sourc e of most, if not all, of the huge far-infrared luminosity of the system. T he optical and K'-band imaging observations and optical spectra suggest mul tiple merger scenarios for one system. If the remaining systems are in an e arly stage of a binary tidal interaction, the commonly accepted interpretat ion of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be reexamined.