Formation of a Tidal Dwarf Galaxy in the Interacting System Arp 245 (NGC 2992/93)

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Abstract
Among the various phenomena observed in interacting galaxies is the ejection due to tidal forces of stellar and gaseous 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 and a numerical model of the collision computed with a Tree-SPH code. Our observations consist of optical/near-infrared broadband imaging, Hα imaging, optical spectroscopy, H I VLA cartography and CO line mapping. The system, composed of the two spiral galaxies NGC 2992 and NGC 2993, is observed at an early stage of the interaction, about 100 Myr after perigalacticon, though at a time when tidal tails have already developed. The VLA observations disclose a third partner to the interaction: an edge-on, flat galaxy, FGC 0938, which looks strikingly undisturbed and might just be falling toward the NGC 2992/93 system. Our H I 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 H I tails emanating from NGC 2993 have a different morphology. In particular, the H I forms a ring, a feature that has been successfully reproduced by our numerical simulations. The H I emission in the system as a whole peaks at the tip of the NGC 2992 tail where a gas reservoir of about 109 M⊙, about 60% of the H I toward 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). Although it is likely, based on our analysis of the H I and model data cube, that A245N might become an independent dwarf galaxy, the dynamical evidence is still open to debate. Prompted by the questions raised for this particular object, we discuss some issues related to the definition and identification of TDGs and highlight some specific conditions which seem required to form them. Finally, we outline what is needed in terms of future numerical simulations in order to further our understanding of these objects.
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