2003ApJ...584..239T

We present here optical spectroscopy and BVRJHK_sphotometry of the recently discovered ultracompact group of galaxies CG J1720-67.8. This work represents a considerable extension of the preliminary results we presented in a previous paper. Despite the complicated morphology of the group, a quantitative morphological classification of the three brightest members of the group is attempted based on photometric analysis. We find that one galaxy is consistent with a morphological type S0, while the other two are most probably late-type spiral galaxies that are already losing their identity because of the interaction process. Information on the star formation activity and dust content derived from both spectroscopic data and optical and near-infrared colors is complemented with a reconstruction of far-infrared maps from IRAS raw data. Enhanced star formation activity is revealed in all the group's members, including the early-type galaxy and the extended tidal tail, along which several tidal dwarf galaxy candidates are identified. The metallicity of the gaseous component is investigated, and photoionization models are applied to the three main galaxies of the group, while a detailed study of the tidal dwarf candidates will appear in a companion paper. Subsolar metal abundances are found for all the three galaxies, the highest values being shown by the early-type galaxy (Z∼0.5 Z_☉).