In 1982 the Canadian Astronomer, Paul Hickson, using the red prints from the Palomar Observatory Sky Survey, published a catalog of 462 galaxies thought to exist within compact groupings. These groupings are now referred to as "Hickson Compact Groups" (HCG). Compact groups are typically four or five galaxies in close physical proximity to one another. They are among the densest concentrations of galaxies known, comparable to the centers of rich galaxy clusters. Compact groups are worthy of intense study as they provide a rich opportunity to study galaxy interactions and merger events. Proof of proximity is difficult and relies on a variety of observational evidence including common redshifts, space distribution, photometry, and other parameters together known as Hickson's isolation criteria.
The true nature of compact groups has been the subject of much controversy. Interpretations have run the gamut from the belief that all represent true physical associations to the belief that they are mere chance alignments. More than likely most are true physically related dynamically bound systems although some are certainly projection artifacts. A recent analysis has shown beyond question that almost half of all HCG galaxies show features indicative of interactions or mergers. This figure is bound to rise with more detailed studies.
Current theory is that compact groups represent physically related galaxies that are in the process of merging into a single object, most likely an elliptical galaxy. Compact groups are relatively short lived entities that form via mergers of galaxies within loose subsystems and groupings. Simulations predict that merging of the group members should proceed rapidly within one billion years to form an elliptical galaxy. Hickson groups are therefore snapshots at various stages in this merging process. The more evolved groups are smaller and more deficient in neutral hydrogen while the least evolved resemble galaxies in looser groups. Observational evidence shows that interaction is occurring in a large fraction of galaxies in HCG supporting this model for the origin and evolution of HCG. The cosmological implications of compact groups are significant. Astronomers believe they may represent an intermediate stage between loose groups and individual galaxies. A better understanding of the nature of HCG could help explain galaxy formation on a larger scale in the early universe.