This
Astrobin Debut Object is a galaxy group of 4 galaxies, called Hickson 41, located in the constellation of Ursa Major at a declination of +45 degrees.
This is not a true cluster - the 4 galaxies are all at much different distances away and are only aligned through superposition from our apparent view.
The visual look of 4 edge-on spirals is striking. Notice that the dimmest edge-on spiral is not included in the definition of the Hickson group. This is because of how Hickson groups are defined in the paper – all of the galaxies must be within 3 orders of brightness magnitude of the brightest (“a”) galaxy. Here, the brightest galaxy is 12.4 magnitude and the dim edge-on spiral is only 16.6 magnitude, so it doesn’t make the criteria to be a member of the group.
The largest and brightest galaxy, Hickson 41a, is also known as UGC 5345. It is located 170 million light years away and spans 1.4 arc-minutes, which corresponds to a diameter of 70,000 light years.
41b is located 340 million light years away, 2x as far away as 41a, and is 100,000 light years in diameter.
41c is located 450 million light years away, almost 3x as far away as 41a, and is 90,000 light years in diameter.
41d, the outlier here, is located 200 million light years away and is only 20,000 light years in diameter.
The dimmest edge-on spiral, not part of the group, is also 200 million light years away and is 50,000 light years in diameter.
The Hickson catalog is a collection of 100 galaxy groups, established by Paul Hickson in 1982. Hickson compact galaxy groups are tightly spaced and somewhat isolated 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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