Astron. Astrophys. 333, 813-826 (1998)

Young tidal dwarf galaxies around the gas-rich disturbed lenticular NGC 5291 ^*

P.-A. Duc ¹ and I.F. Mirabel <sup>2

1</sup> ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
² CEA, SAp, C.E. Saclay, F-91191 Gif-sur-Yvette Cedex, France &
Instituto de Astronomía y Física del Espacio, Argentina

Received 25 September 1997 / Accepted 9 December 1997

Abstract

NGC 5291 is an early type galaxy at the edge of the cluster Abell 3574 which drew the attention because of the unusual high amount of atomic gas () found associated to it. The HI is distributed along a huge and fragmented ring, possibly formed after a tidal interaction with a companion galaxy. We present multi-slit optical spectroscopic observations and optical/near-infrared images of the system. We show that NGC 5291 is a LINER galaxy exhibiting several remnants of previous merging events, in particular a curved dust lane and a counter-rotation of the gas with respect to the stars. The atomic hydrogen has undoubtly an external origin and was probably accreted by the galaxy from a gas-rich object in the cluster. It is unlikely that the HI comes from the closest companion of NGC 5291, the so-called "Seashell" galaxy, which appears to be a fly-by object at a velocity greater than 400 km s^-1.

We have analyzed the properties of 11 optical counterparts to the clumps observed in the HI ring. The brightest knots show strong similarities with classical blue compact dwarf galaxies. They are dominated by active star forming regions; their most recent starburst is younger than 5 Myr; we did not find evidences for the presence of an old underlying stellar population. NGC 5291 appears to be a maternity of extremely young objects most probably forming their first generation of stars. Born in pre-enriched gas clouds, these recycled galaxies have an oxygen abundance which is higher than BCDGs ( on average) and which departs from the luminosity-metallicity relation observed for typical dwarf and giant galaxies. We propose this property as a tool to identify tidal dwarf galaxies (TDGs) among the dwarf galaxy population. Several TDGs in NGC 5291 exhibit strong velocity gradients in their ionized gas and may already be dynamically independent galaxies.

Key words: galaxies: individual: NGC 5291 galaxies: interactions galaxies: formation galaxies: ISM galaxies: dwarf radio lines: galaxies

* Based on observations collected at the European Southern Observatory, La Silla, Chile. ESO No 53.1-076 and ESO No 56.A-0757

Send offprint requests to: pduc@eso.org

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Contents

• 1. Introduction
• 2. Observations and data-reduction
  • 2.1. Optical and near-infrared imaging
  • 2.2. Optical spectroscopy
• 3. Results
  • 3.1. NGC 5291
  • 3.2. The Seashell
  • 3.3. The optical knots associated with the HI ring
  • 3.4. Other objects
• 4. Discussion
  • 4.1. Properties of the optical knots
    • 4.1.1. Metallicity
    • 4.1.2. Extinction
    • 4.1.3. Current star formation
    • 4.1.4. Stellar populations
    • 4.1.5. Dynamics
  • 4.2. Origin of the HI in NGC 5291
  • 4.3. Tidal dwarf galaxies
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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