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Issue A&A
Volume 425, Number 3, October III 2004
Page(s) 813 - 823
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20040394



A&A 425, 813-823 (2004)
DOI: 10.1051/0004-6361:20040394

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

F. Bournaud1, 2, 3, P.-A. Duc1, 4, P. Amram5, F. Combes2 and J.-L. Gach5

1  CEA/DSM/DAPNIA, Service d'Astrophysique, Saclay, 91191 Gif-sur-Yvette Cedex, France
    e-mail: Frederic.Bournaud@obspm.fr
2  Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014, Paris, France
3  École Normale Supérieure, 45 rue d'Ulm, 75005, Paris, France
4  CNRS FRE 2591
5  Observatoire Astronomique Marseille-Provence & Laboratoire d'Astrophysique de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 04, France

(Received 5 March 2004 / Accepted 4 June 2004)

Abstract
The kinematics of tidal tails in colliding galaxies has been studied via Fabry-Pérot observations of the H $\alpha$ 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 10 9 $M_{\odot}$ 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".


Key words: galaxies: interaction -- galaxies: formation -- galaxies: evolution -- galaxies: kinematics and dynamics

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© ESO 2004


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