At one time it was thought that these groups were merely chance projections on the sky. However, recent research has shown that most are indeed gravitationally bound.
|For comparison, the Leo group is a loose group of galaxies. Note the large separations of galaxies compared to the three Hickson Compact Groups.||
The velocities measured for galaxies in compact groups are quite low (~200 km/s), making these environments highly conducive to interactions and mergers between galaxies. However, this makes the formation of compact groups something of a mystery, as the close proximity of the galaxies means that they should merge into a single galaxy in a short time, leaving only a fossil group. This would mean that compact groups are a shorted-lived phase of group evolution, and we would expect them to be extremely rare. Instead, we find a significant number of compact groups in the nearby Universe, with well over 100 identified.
Two ideas proposed to explain the high abundance of compact groups are:
While it is not yet clear why we observe so many compact groups, research continues to try to unravel the mystery.