Formation of a tidal dwarf galaxy in the interacting system Arp 245 (NGC 2992/93)

Among the various phenomena observed in interacting galaxies is the ejection due to tidal forces of material into the intergalactic medium and its subsequent rearranging which can lead to the formation of self-gravitating tidal dwarf galaxies (TDGs). We investigate this process with a detailed multiwavelength study of the interacting system Arp 245 (NGC 2992/3) and a numerical model of the collision. The system is observed at an early stage of the interaction, about 100 Myr after perigalacticon, though at a time when tidal tails have already developed. Our HI map shows prominent counterparts to the optical tails. Whereas the stellar and gaseous components of the plume that originates from NGC 2992 match, the stellar and HI tails emanating from NGC 2993 have a different morphology. In particular, the HI forms a ring, a feature that has been successfully reproduced by our numerical simulations. The HI emission in the system as a whole peaks at the tip of the NGC 2992 tail where a gas reservoir of about 10^9 Msun, about 60% of the HI towards NGC 2992, coincides with a star-forming optical condensation, A245N. The latter tidal object exhibits properties ranging between those of dwarf irregular galaxies (structural parameters, gas content, star formation rate) and those of spiral disks (metallicity, star formation efficiency, stellar population). Prompted by the questions raised for this particular object, we discuss some issues related to the definition and identification of TDGs (abridged)