This
Astrobin Debut Object is a galaxy group of 4 galaxies, called Hickson 42, located 200 million light years away in the constellation of Hydra at a declination of -20 degrees.
This is a true cluster - all 4 galaxies are about the same distance away.
The largest and brightest galaxy, Hickson 42a, is also known as NGC 3091. It is an E3 elliptical that spans 3 arc-minutes, which corresponds to a large diameter of 160,000 light years. This is one of the brightest Hickson galaxies at a magnitude of 10.9.
Hickson 42b has the most interesting structure of the group. It is a barlense ring spiral galaxy.
Just below and right of 42b is a small galaxy which seems like it should have been included as 42e, but Hickson did not include this galaxy in the group. It likely is not quite bright enough, as it falls just outside the criteria of being within 3 magnitudes of the brightest galaxy in the group (42a).
The Hickson catalog is a collection of 100 galaxy groups, established by Paul Hickson in 1982. Hickson compact galaxy groups are strictly defined by 3 criteria: number of galaxies, total magnitude, and isolation from other galaxies. A typical Hickson group has 4 galaxies, but some have up to 8. You can see more about Hickson galaxy groups in my
Astrobin Hickson Collection.
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