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

Abstract

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 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 a sequence in evolution, which is correlated with the spatial configuration of the galaxies making up a CG. We distinguish three main configuration: Type A CGs show predominantly low velocity dispersions and are rich in late-type spirals that are active in terms of 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 is formed by CGs with high velocity dispersions, which are dominated by elliptical galaxies that show no activity. We suggest that the level of evolution increases in the sense A-->B-->C. 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. Instead, we observe a trend that goes contrary to expectation: the level of evolution 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 level of evolution 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 CDM (or Lambda CDM) structures formation scenarios, assuming the formation of galaxies is a biased process.