Compact Groups in the UZC galaxy sample : II . Connections between morphology , luminosity and large - scale density ⋆

The nature of Compact Groups (CGs) is investigated by comparing luminosity and morphology of CG galaxies, CG Neighbours and Isolated galaxies. CGs turn out to display a clear excess of both E-S0 and bright galaxies with respect to Isolated galaxies. Neighbours exhibit luminosity and morphological content intermediate between Isolated and CG galaxies. Differences between CGs and their surrounding Neighbours do however cancel out when dominant galaxies (33 E-S0 and 30 Spirals) are excluded from the CG sample. This indicates it is not the whole CG population that is peculiar, but rather, that CGs typically include a dominant galaxy whose presence strongly biases CGs average properties When this galaxy is an Elliptical it dominates also over its large scale (1h −1 Mpc) Neighbours, a result which holds true for only half of the dominant Spirals. Relations linking the number of Neighbours of dominant E-S0s to the luminosity of E-S0s and to the difference between the first and the second ranked CG members do suggest a link to exist between the formation of bright early-type galaxies and the presence of a group-like potential. No similar relations are found for dominant Spirals, indicating they are either projections, or brightest galaxies of small systems. These findings, coming from the analysis of an optically selected CG sample, correspond to analogous results derived from the X-analysis of groups.