Kinematics of tidal tails in interacting galaxies: Tidal dwarf galaxies and projection effects

The kinematics of tidal tails in colliding galaxies has been studied via Fabry-Pérot observations of the Hα emission. With their large field of view and high spatial resolution, the Fabry-Pérot 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 (10$^9~M_{\odot}$) 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 50–100 km s-1 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-Pérot observations have thus enabled us to prove that some 109 $M_{\odot}$ 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”