The Hickson catalog is a collection of 100 galaxy groups established by Paul Hickson in 1982. The criteria for a Hickson Galaxy Group is specific and detailed in
his paper, but basically it boils down to 3 simple things:
1. The group must have at least 4 galaxies, all with decent brightness.
2. The group must be isolated and not just part of a large cluster.
3. The group has to be compact, with spacing between them similar to their sizes.
Hickson groups range from 4 to 8 galaxies. The galaxies are generally small, distant and a challenge to image. The diameter of a circle which surrounds each entire group ranges from 1 arc-minute to 16 arc-minutes. The brightest galaxy in each group is usually about magnitude 14.
The technical data for these 18 galaxies is shown in Revision D, including location, distance and size. The Hickson catalog is biased to the northern hemisphere and that bias is also reflected in the short list above.
Galaxies in a Hickson group are often a true group and are the same distance away, but there are exceptions. The most notable exception is the large blue galaxy in Hickson 61 on the poster, which is 3x closer to us than its 3 "companions".
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