Compact Groups of Galaxies
Galaxies are sociable, so to speak, as most of them actually live in the group or cluster environment. The most “communicative” kind of group are “compact groups”.
Compact groups were defined in 1982 by Paul Hickson as galaxy groups with at least four bright (massive) members and a total luminosity of more than 26 Magnitudes, that are isolated from any other surrounding structure.
We still use this criterion today to identify compact groups. In many compact groups, the galaxies show signs of interactions, but there are also several compact groups where the galaxies are aligned along a “string” like pearls.
Several of such groups were already included by Halton Arp in his “Atlas of Peculiar Galaxies”. The best known example of a compact group is “Stephan’s Quintet”.
Compact groups are very interesting objects, because they provide the possibility to study the different stages of galaxy interactions in detail, from simple gravitational interactions causing warps and stellar streams, to complex interactions of the gaseous components causing shocks and starbursts.
Even nowadays, the lifetimes and evolutionary stages of compact groups are still not fully understood, and are thus exciting objects for theorists and observers.