This
Astrobin Debut Object is a galaxy group of 4 galaxies, called Hickson 45, located 1 billion light years away in the constellation of Ursa Major at a declination of +59 degrees.
This is a true cluster - the 4 galaxies are about the same distance away. At 1 billion light years away, it is too difficult to tell whether they are interacting with each other.
You may have noticed that the bright edge-on galaxy to the lower right is not included in the Hickson 45 group. Although this galaxy is the same distance away as the other 4, it lies just far enough away from the other 4 that it is excluded from the group based on the Hickson criteria for galaxy proximity with each other.
The largest and brightest galaxy, Hickson 45a, is also known as UGC 5564. It is a magnitude 14 spiral galaxy that spans 1.1 arc-minutes, which corresponds to a huge diameter of 290,000 light years. This is one of the largest galaxies that I have ever imaged in terms of actual size. In comparison, the other spiral galaxies in the frame are Milky Way size, at about 120,000 light years in diameter. I find the core of 45a to look a bit odd, offset slightly up from the midline, but that is the same way it looks on the rough sky atlas DSS image so I think it is a real thing and not some kind of artifact.
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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