Gas dynamics in tidal dwarf galaxies: Disc formation at z = 0^{⋆,⋆⋆,⋆⋆⋆}

¹ Astronomy Department, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
e-mail: federico.lelli@case.edu
² Laboratoire AIM, CNRS, CEA/DSM, Université Paris Diderot, 91191 Gif-sur-Yvette, France
³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
⁴ Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁵ Leibniz–Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Laboratoire d’Astrophysique de Marseille, Observatoire Astronomique Marseille–Provence, Université de Provence & CNRS, 2 place Le Verrier, 13248 Marseille Cedex 4, France
⁷ Unidad de Astronomia, Facultad de Ciencias Basicas, Universidad de Antofagasta, Avenida Angamos 601, 02800 Antofagasta, Chile
⁸ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁹ Laboratoire d’Astrophysique de Bordeaux, Observatoire de Bordeaux, 2 rue de l’Observatoire, 33270 Floirac, France
¹⁰ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia

Received: 27 May 2015
Accepted: 17 September 2015

Abstract

Tidal dwarf galaxies (TDGs) are recycled objects that form within the collisional debris of interacting and merging galaxies. They are expected to be devoid of non-baryonic dark matter, since they can only form from dissipative material ejected from the discs of the progenitor galaxies. We investigate the gas dynamics in a sample of six bona fide TDGs around three interacting and post-interacting systems: NGC 4694, NGC 5291, and NGC 7252 (“Atoms for Peace”). For NGC 4694 and NGC 5291, we analyse existing H I  data from the Very Large Array (VLA), while for NGC 7252 we present new H I   observations from the Jansky VLA, together with long-slit and integral-field optical spectroscopy. For all six TDGs, the H I  emission can be described by rotating disc models. These H I  discs, however, have undergone less than a full rotation since the time of the interaction/merger event, raising the question of whether they are in dynamical equilibrium. Assuming that these discs are in equilibrium, the inferred dynamical masses are consistent with the observed baryonic masses, implying that TDGs are devoid of dark matter. This puts constraints on putative “dark discs” (either baryonic or non-baryonic) in the progenitor galaxies. Moreover, TDGs seem to systematically deviate from the baryonic Tully-Fisher relation. These results provide a challenging test for alternative theories like MOND.