5 Conclusions

Deep optical images, spectra and high-resolution NIR images are presented for IRAS03158+4227, one of the most luminous ULIRGs of the 2 Jy sample. The host galaxy, G1, is identified with a component of a binary of nearly equal-luminosity giant galaxies separated by $47\,h_{75}^{-1}$kpc. The companion galaxy, G2, harbours an active nucleus and has an extended, curved tail. This tail may be interpreted as a tidal structure induced by the gravitational interaction with the host galaxy of the ULIRG. The high-resolution NIR images do not reveal any sign for a close double nucleus down to a (resolution-limited) nuclei separation of 1 kpc.

ULIRGs in such widely separated systems may be explained by a multiple merger, like IRAS14394+532 (Dinh-V-Trung et al. 2001), where the interaction of two components has already reached an advanced stage with a very small separation of nuclei. Indeed, the optical luminosity profile of G1 is reasonably fit by a deVaucouleurs law. The profile of G2 is more compatible with a disk-like structure. Hence it seems likely, in the multi-merger scenario, that the ULIRG in G1 was triggered by a past merger and the AGN in G2 by the present interaction between G1 and G2. If the nuclear activity of G2 is also due to a (late) merger, the system IRAS03158+4227 would represent the case of the gravitational interaction of two advanced mergers. Such scenarios would imply that multiple mergers in compact groups can trigger very intense starbursts.

As an alternative explanation, both the ULIRG IRAS03158+4227 and the nuclear activity in its neighbour galaxy might be dynamically triggered by the same process, namely the gravitational interaction of these two galaxies. In this case, IRAS03158+4227 would represent an early stage of binary tidal interaction, and the interpretation of ULIRGs as final merger stages may need to be reexamined.

Acknowledgements

This research is based on observations made with the 3.5 m telescope and the 2.2 m telescope of the German-Spanish Astronomical Centre, Calar Alto, Spain. We acknowledge financial support from the Deutsche Forschungsgemeinschaft under grants Me1350/3, 5, 8, 14 and Ste605/15. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.