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Tidal Debris Posing as Dark Galaxies

Published online by Cambridge University Press:  01 June 2007

Pierre–Alain Duc ,
Frédéric Bournaud  and
Elias Brinks
Pierre–Alain Duc
Affiliation:
Laboratoire AIM, DSM/CEA – CNRS – Université Paris Diderot, DAPNIA/SAp, CEA–Saclay, 91191 Gif sur Yvette cedex, France email: paduc@cea.fr
Frédéric Bournaud
Affiliation:
Laboratoire AIM, DSM/CEA – CNRS – Université Paris Diderot, DAPNIA/SAp, CEA–Saclay, 91191 Gif sur Yvette cedex, France email: paduc@cea.fr
Elias Brinks
Affiliation:
Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK email: E.Brinks@herts.ac.uk
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Abstract

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Debris sent into the intergalactic medium during tidal collisions has received much attention as it can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons.

High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star–less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter (DM) dominated, objects which for some reason never managed to form stars. VirgoHI21, in the Virgo Cluster, is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly–by at about 1000 km s−1 by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects (Duc & Bournaud, 2007).

Based on our multi-wavelength and numerical studies of galaxy collisions, we discuss several ways to identify a tidal origin in a Dark Galaxy candidate such as optical and millimetre–wave observations to reveal a high metallicity and CO lines, and more importantly, kinematics indicating the absence of a prominent Dark Matter halo. We illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy (Duc et al., 2007).

Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 (Bournaud et al., 2007) and probably in the TDG VCC 2062. These “Lost Baryons” must originally have been located in the disks of their parent galaxies.

Keywords

Galaxies:interactionsgalaxies:peculiargalaxies: dwarfgalaxies: clusters: individual: Virgogalaxies: individual (VirgoHI21, NGC 4254, NGC 4694, VCC 2062)(cosmology:) dark matter
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 3 , Symposium S244: Dark Galaxies and Lost Baryons , June 2007 , pp. 216 - 225
Copyright
Copyright © International Astronomical Union 2008

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