4 Summary and discussion

With the aim of studying the effect of the environment on the physics of early-type galaxies, we have started to collect relevant spectroscopic data, and this paper presents kinematical results for seven galaxies in the Hickson Compact Groups 67, 74, and 79. Stellar kinematics are seldom available for members of such groups, and our modest sample already represents a substantial fraction of the amount available in the literature. All the groups examined display very compact configurations in the sky, are mainly composed of early-type galaxies and, according to the literature, show several morphological signature of interaction. One of them, HCG 67, is embedded withing an X-ray diffuse background while HCG 74 and HCG 79 have only an upper limit measured (Ponman et al. [1996]).

We have looked for perturbations in the kinematical profiles, and we have carefully examined broad-band images in search of morphological peculiarities which could possibly be associated to these perturbations. In this task, we have attributed more weight to the shape of the rotation-velocity curve than to the velocity-dispersion one, as likely to be more unambiguously connected to gravitational interactions (Combes et al. [1995]). Our measurements show several cases of apparent kinematical interactions, but for all of them, the mere optical superposition cannot be ruled out as the explanation.

Morphological evidence of physical interaction is sometimes elusive, and, for example, the reality of multiple nuclei also suffers from the ambiguity of the position along the same line of sight. For HCG 67, we have been able to show that the putative second nucleus is actually another, smaller galaxy; there is no compelling evidence that it is undergoing a merging process. The fine structure of the central galaxy is, for this group, the strongest optical indication of a connection with the neighborhood. Convincing signature of severe gravitational perturbation is provided by warped disks and irregular extensions, but they cannot be considered as more than mild evidence of interaction within a physical group.

Rampazzo et al. ([1998]) noticed that morphological signature, kinematical features and the presence of hot gas are poorly correlated with one another. This also emerges from the present study. We consider the lack of compelling results in the present paper as an incentive to widen our sample and diversify our approach. We will gather spectroscopic and photometric data in compact systems, and also in looser ones, where superposition effects would be less severe. We plan to pay attention, again, to kinematical signatures if they appear unambiguous, but also to scaling relations which would link together several photometric and dynamical aspects. For this, a significant sample is needed, and we consider the present one as a first step.

Acknowledgements

We are deeply indebted to Daniela Bettoni for the permission to inspect her imaging material and kinematic data on HCG 67 before publication; we are also very grateful to Paul Hickson for the permission to use his CFHT images. We thank the referee, Giovanni Busarello, for his very careful and detailed comments on the initial version of the paper. We thank Gregory Maubon, and also the telescope operators of the Observatoire de Haute-Provence for their help in securing the observations.