10 Conclusions

We have taken advantage of a large, almost complete 3D catalogue to identify a sample of 291 northern CGs with redshift between 1000 and 10 000 km s$^{\rm -1}$. CGs include a minimum of 3 members which have to lie within a region of 200 h^-1 kpc and $\Delta v=\pm 1000$ km s^-1. Kinematical properties of CGs and spectral characteristics of member galaxies have been investigated and related to large-scale environmental parameters. The sample has been used to compare Triplets, which constitute 76% of the sample, to higher-multiplicity structures. The analysis indicates that multiplicity is intrinsically linked to CG properties such as velocity dispersion, large-scale environment and spectral characteristics of galaxies. Specifically, it is found that Ts are more likely to be isolated systems and to display low velocity dispersion as well as a high gas-rich galaxy content. We suggest that Ts, although affected by interlopers, generally correspond to field galaxy structures. They constitute ideal sites for efficient merging to occur, and are thus likely to transform into a single galaxy as continuous accretion from surrounding galaxies is not viable on times shorter than their dynamical time scale.

On the other hand, higher multiplicity CGs are mainly associated with high velocity dispersion systems, whose members are preferentially gas-poor galaxies. These CGs display lower density contrast than field CGs and may thus suffer contamination by systems that are just temporary chance alignments within loose groups/poor clusters. Those Ms which are real physical systems should constitute the center of a larger collapsing group and are thus expected to display diffuse X-ray emission.

In summary, our data indicate that, provided most CGs are real physical systems, Ts and Ms correspond to two extremely different classes of systems. Therefore, any fair analysis of CGs properties should treat Ms and Ts separately.

Acknowledgements

We are pleased to thank S. Bardelli, A. Cappi, S. Giovanardi, P. Hickson, A. Iovino, G. G. C. Palumbo, E. Rossetti, R. Sancisi, G. Stirpe and V. Zitelli for stimulating discussions and suggestions. We thank the referee G. Mamon whose comments and criticism greatly improved the scientific content of the paper. This work was supported by MURST. B.K. acknowledges a fellowship of Bologna University.