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 presumably in the early phase of interaction. This fact is contrary to the conventional 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) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected CO emission in five out of six systems, but only in one component of the ULIRG pairs. Four of them have LINER spectral type and one galaxy has Seyfert 2 spectral type. In K′-band images these components are also brighter than the other components which have either H II region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 1010 M☉, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K′-band imaging observations and optical spectra suggest multiple merger scenarios for one system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be reexamined.