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 ULIRGs from the IRAS 2 Jy sample. Deep optical images are presented along with 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 the companion, which harbours an active nucleus, is the most prominent peculiar feature of the binary. The results of numerical simulations permit the interpretation of this structure as a product of the tidal interaction between the two components. If the infrared-activity of IRAS 03158+4227 is also dynamically triggered by this process, this would imply that the ULIRG phenomenon is not restricted to the final stage of a binary merger. Alternatively, the system may be a multiple merger where the partner(s) has/have already coalesced.