The formation of tidal dwarf galaxies in interacting systems: The case of Arp 245 (NGC 2992/93)

We present some highlights of our multi-wavelength study, which involves op tical broad- and narrow-band imaging, long-slit spectroscopy, high-resoluti on HI and CO observations, of the interacting system Arp 245. This object c onsists of the galaxies NGC 2992 and NGC 2993. Based on a numerical model o f the collision, which was computed with a Tree-SPH code, we derive that Ar p 245 is observed at an early stage of the interaction, about 100 Myr after perigalacticon, though at a time when tidal tails have already developed. At the tip of the NGC 2992 tail we find a gas reservoir of about 10(9) M-., or about 60% of the HI which is seen towards NGC 2992, which coincides wit h what appears to be a star-forming tidal dwarf galaxy, A245N. The TDG A245 N exhibits properties ranging between those of dwarf irregular galaxies (st ructural parameters, gas content, star formation rate) and those of spiral disks (metallicity, star formation efficiency, stellar population). We spec ulate what the required conditions are to form a TDG, and how they can be d istinguished from field dwarf irregulars.