The relation between galaxy activity and the dynamics of compact groups of galaxies

Using a sample of 91 galaxies distributed over 27 compact groups (CGs) of galaxies, we define an index that allows us to quantify their level of activity due to an active galactic nucleus (AGN) or star formation. By combining the mean activity index with the mean morphological type of the galaxies in a group, we are able to quantify the evolutionary state of the groups. We find that they span an evolutionary sequence that correlates with the spatial configuration of the galaxies in the CG. We distinguish three main configuration types: A, B, and C. Type A CGs show predominantly low velocity dispersions and are rich in late-type spirals that show active star formation or harbor an AGN. Type B groups have intermediate velocity dispersions and contain a large fraction of interacting or merging galaxies. Type C comprises CGs with high velocity dispersions, which are dominated by elliptical galaxies that show no activity. We suggest that evolution proceeds Adouble right arrowBdouble right arrowC. Mapping the groups with different evolution levels in a diagram of radius versus velocity dispersion does not reveal the pattern expected based on the conventional fast merger model for CGs, which predicts a direct relation between these two parameters. Instead, we observe a trend contrary to expectation: the evolutionary state of a group increases with velocity dispersion. This trend seems to be related to the masses of the structures in which CGs are embedded. In general, the evolutionary state of a group increases with the mass of the structure. This suggests either that galaxies evolve more rapidly in massive structures or that the formation of CGs embedded in massive structures predated the formation of CGs associated with lower mass systems. Our observations are consistent with the structure formation predicted by the CDM model (or LambdaCDM), only if the formation of galaxies is a biased process.