2004A&A...425..813B

The kinematics of tidal tails in colliding galaxies has been studied via Fabry-Perot observations of the Hα emission. With their large field of view and high spatial resolution, the Fabry-Perot data allow us to probe simultaneously, in 2D, two kinematical features of the tidal ionized gas: large-scale velocity gradients due to streaming motions along the tails, and small-scale motions related to the internal dynamics of giant HII regions within the tails. In several interacting systems, massive (10⁹M_☉) condensations of HI, CO and stars are observed in the outer regions of tails. Whether they are genuine accumulations of matter or not is still debated. Indeed a part of the tidal tail may be aligned with the line-of-sight, and the associated projection effect may result in apparent accumulations of matter that does not exist in the 3D space. Using numerical simulations, we show that studying the large-scale kinematics of tails, it is possible to know whether these accumulations of matter are the result of projection effects or not. We conclude that several ones (Arp 105-South, Arp 242, NGC 7252, and NGC 5291-North) are genuine accumulations of matter. We also study the small-scale motions inside these regions: several small-scale velocity gradients are identified with projected values as large as 50-100km/s accross the observed HII regions. In the case of NGC 5291-North, the spatial resolution of our observations is sufficient to detail the velocity field; we show that this system is rotating and self-gravitating, and discuss its dark matter content. The Fabry-Perot observations have thus enabled us to prove that some 10⁹M_☉ condensations of matter are real structures, and are kinematically decoupled from the rest of the tail. Such massive and self-gravitating objects are the progenitors of the so-called ``Tidal Dwarf Galaxies''.