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 ejectiondue to tidal forces of material into the intergalactic medium and itssubsequent rearranging which can lead to the formation of self-gravitatingtidal dwarf galaxies (TDGs). We investigate this process with a detailed multiwavelength study ofthe interacting system Arp 245 (NGC 2992/3) and a numerical model of thecollision. The system is observed at an early stage of the interaction, about 100 Myrafter perigalacticon, though at a time when tidal tails have already developed. Our HI map shows prominent counterparts to the optical tails. Whereas thestellar and gaseous components of the plume that originates from NGC 2992match, the stellar and HI tails emanating from NGC 2993 have a differentmorphology. In particular, the HI forms a ring, a feature that has beensuccessfully reproduced by our numerical simulations. The HI emission in thesystem as a whole peaks at the tip of the NGC 2992 tail where a gas reservoirof about 10^9 Msun, about 60% of the HI towards NGC 2992, coincides with astar-forming optical condensation, A245N. The latter tidal object exhibits properties ranging between those ofdwarf irregular galaxies (structural parameters, gas content, star formationrate) and those of spiral disks (metallicity, star formation efficiency, stellar population). Prompted by thequestions raised for this particular object, we discuss some issues related tothe definition and identification of TDGs (abridged)Comment: 43 pages, 20 figures, Astronomical Journal, in pres