Galaxy groups are the smallest assemblages of galaxies in the universe, ranking in size below clusters and superclusters. Compact groups are a sub-class of the broader category, comprising groups in which the distance between the member galaxies is roughly of the same order of magnitude as the size of the galaxies themselves. Several catalogues of compact galaxy groups have been constructed, but the most widely utilized is that developed in 1982 by Canadian astronomer Paul Hickson.

This is an image of the 44th entry in Hickson's catalogue of compact galaxy groups. Located in the constellation Leo, HCG 44 lies about 80 million years light away. Located near the center of the four-galaxy group is edge-on spiral NGC 3190. To its lower left is elliptical galaxy NGC 3193. At the top center of the image is barred spiral galaxy NGC 3187. Finally, to the upper right is barred spiral NGC 3185.

As is typically the case in compact groups, all members of HCG 44 show signs of gravitational interaction with each other. Especially prominent in this regard is the dim halo around NGC 3190, which can be seen stretching in the direction of NGC 3193 and, less distinctly in my image, toward NGC 3187.

The extensive interaction between members of compact groups poses a problem for astrophysics. Given the degree to which they interact with each other, we would expect members of compact groups to undergo numerous, frequent mergers with each other, with the ultimate result being that the group would disappear, to be replaced by a single, large elliptical galaxy. We would therefore expect compact galaxy groups to be relatively rare in the local universe. However, it turns out that such groups are plentiful. (Hickson's catalogue includes 100 of them.) Two different explanations are offered to account for this phenomenon. It has been hypothesized that compact groups might be more dynamically stable than we would expect, perhaps as a result of being surrounded by a single, large halo of dark matter. Alternatively, it has been hypothesized that a compact stage represents the final phase of evolution for all galaxy groups. If such were the case, compact groups passing out of existence in a flurry of mergers would be replaced by new groups entering the compact phase as the final stage in their evolution.