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

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

Corresponding author: M. G. Richer, richer@astrosen.unam.mx

Received: 21 August 2001
Accepted: 23 September 2002

Abstract

We present Fabry-Perot and multi-object spectroscopy of the galaxies in Hickson compact group 31 (HCG 31). Based upon our Hα 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 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.