Morphology of Galaxies in Compact Groups

We present the results of an isophotal analysis of 140 early-type galaxies and a visual inspection of images of an additional 202 galaxies in compact groups. This is essentially the entire sample of galaxies in the subset of 92 Hickson compact groups which have at least three accordant members. About 12% of the elliptical galaxies have larger characteristic radii and shallower surface brightness profiles than galaxies of the same luminosity in less dense environments. The average ellipticity of elliptical galaxies in compact groups is a slowly increasing function of the metric radius, as it is for field and loose- group galaxies. No alignment is found among the major axes of the galaxies and the major axis of the group. When combined with previously published morphological, kinematic, radio, infrared, and color information on the same galaxies, our data show that 43% of the galaxies in the compact group sample show morphological and/or kinematical distortions indicative of interactions and/or mergers. About 32% of the groups have three or more galaxies which show some sign of interaction. This is a lower limit, since for the great majority of the galaxies in the groups, only imaging and low-resolution spectra are available. For the subsample of 16 groups for which published detailed kinematical data are also available, the fraction of groups with three or more galaxies in interaction is 75%. No correlation is found between the number of interacting galaxies in a group and the group velocity dispersion or crossing time. These observations strongly support the view that compact groups are systems of physically associated galaxies and not chance alignments of field, loose-group, or cluster galaxies. They also confirm the importance of compact groups for studies of interactions and galaxy evolution. While the lack of a good control sample makes it difficult to make quantitative comparisons for some aspects of this study, it is clear that the fraction of galaxies showing evidence of interactions is much higher in compact groups than in other environments.