The merger stage of the ultra-luminous infrared galaxy IRAS 03158+4227

We examine the merger stage of IRAS 03158+4227, one of the most luminous UL IRGs from the IRAS 2 Jy sample. Deep optical images are presented along wit h high-resolution NIR images and optical low-resolution spectra. IRAS 03158 +4227 is confirmed as a component of an equal-luminosity binary galaxy with a nuclei separation of 47 h(75)(-1). A long lopsided tail emanating from t he companion, which harbours an active nucleus, is the most prominent pecul iar feature of the binary. The results of numerical simulations permit the interpretation of this structure as a product of the tidal interaction betw een the two components. If the infrared-activity of IRAS 03158+4227 is also dynamically triggered by this process, this would imply that the ULIRG phe nomenon is not restricted to the final stage of a binary merger. Alternativ ely, the system may be a multiple merger where the partner(s) has/have alre ady coalesced.