The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies

Using a sample of 91 galaxies distributed over 27 Compact Groups of Galaxies(CGs), we define an index that allows us to quantify their level of activity,be it AGN or star formation. By combining the mean activity index with the meanmorphological type of the galaxies in a group we are able to quantify theevolutionary state of the groups. We find that they span a sequence inevolution, which is correlated with the spatial configuration of the galaxiesmaking up a CG. We distinguish three main configuration: Type A CGs showpredominantly low velocity dispersions and are rich in late-type spirals thatare active in terms of star formation or harbor an AGN; Type B groups haveintermediate velocity dispersions and contain a large fraction of interactingor merging galaxies; Type C is formed by CGs with high velocity dispersions,which are dominated by elliptical galaxies that show no activity. We suggestthat the level of evolution increases in the sense A-->B-->C. Mapping thegroups with different evolution levels in a diagram of radius versus velocitydispersion does not reveal the pattern expected based on the conventional fastmerger model for CGs. Instead, we observe a trend that goes contrary toexpectation: the level of evolution of a group increases with velocitydispersion. This trend seems to be related to the masses of the structures inwhich CGs are embedded. In general, the level of evolution of a group increaseswith the mass of the structure. This suggests either that galaxies evolve morerapidly in massive structures or that the formation of CGs embedded in massivestructures predated the formation of CGs associated with lower mass systems.Our observations are consistent with CDM (or Lambda CDM) structures formationscenarios, assuming the formation of galaxies is a biased process.Comment: Accepted for publication in The Astronomical Journa