EDP Sciences
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Volume 397, Number 1, January I 2003
Page(s) 99 - 107
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021394

A&A 397, 99-107 (2003)
DOI: 10.1051/0004-6361:20021394

Is HCG 31 undergoing a merger or a fly-by interaction?

M. G. Richer1, L. Georgiev2, M. Rosado2, A. Bullejos3, 2, M. Valdez-Gutiérrez4, 5 and D. Dultzin-Hacyan2

1  Observatorio Astronómico Nacional, Instituto de Astronomía, UNAM, PO Box 439027, San Diego, CA 92143-9027, USA
2  Instituto de Astronomía, UNAM, Apartado Postal 70-264, Ciudad Universitaria, 04510 México, D.F., México
    e-mail: [georgiev, margarit, deborah]@astroscu.unam.mx
3  Instituto de Astrofísica de Canarias, Vía Láctea, 38200 La Laguna, Tenerife, Spain
    e-mail: almudena@astroscu.unam.mx
4  LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
    e-mail: margarita.valdez@obspm.fr
5  Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51, 72000 Puebla, Puebla, México
    e-mail: mago@inaoep.mx

(Received 21 August 2001 / Accepted 23 September 2002 )

We present Fabry-Perot and multi-object spectroscopy of the galaxies in Hickson compact group 31 (HCG 31). Based upon our H $\alpha$ data cubes, galaxies A and C are a single entity, showing no discontinuity in their kinematics. Kinematically, galaxy E is probably a component of the A+C complex; otherwise it is a recently detached fragment. Galaxy F appears, both kinematically and chemically, to have formed from material tidally removed from the A+C complex. Galaxies B and G are kinematically distinct from this complex. Galaxy Q also has a radial velocity compatible with group membership. Galaxies A, B, C, and F have nearly identical oxygen abundances, despite spanning a luminosity range of 5 $\,$mag. Galaxy B's oxygen abundance is normal for its luminosity, while galaxy F's abundance is that expected given its origin as a tidal fragment of the A+C complex. The oxygen abundances in galaxies A and C are also understandable if the A+C complex is a late-type spiral suffering strong gas inflow and star formation as a result of a tidal interaction. Given the kinematics of both the galaxies and the $\ion{H}{i}$ gas, the oxygen abundances, and the position of galaxy G, we propose that an interaction of galaxy G with the A+C complex, rather than a merger of galaxies A and C, is a more complete explanation for the tidal features and other properties of HCG 31. In this case, the A+C complex need not be a merger in progress, though this is not ruled out.

Key words: galaxies: abundances -- galaxies: evolution -- galaxies: formation -- galaxies: individual: NGC 1741 -- galaxies: clusters: individual: HCG 31

Offprint request: M. G. Richer, richer@astrosen.unam.mx

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© ESO 2003