Much has been written about 3 arm spiral galaxies over the years, starting with the famous 3 arm categorization that Dr. Halton Arp used in his Arp Catalogue, published in 1966. Dr. Arp identified three 3 arm galaxies - Arp 19, Arp 20 and Arp 21.
But are 3 arm galaxies a real thing? Several papers have been written saying that they are, even though their formation mechanism is unexplainable using current theories of spiral structure.
Based on my experience, my view is no, there is no such thing as a true 3 arm galaxy. Most spiral galaxies I have imaged have 2 arms, a few seem to have 4, while others are so fragmented or flocculent that it is hard to tell. This latter group includes galaxies which have ragged disks and disrupted arm structures. Especially when viewed at a distance, some of these have the appearance of 3 arms in the outer disk. But when examined carefully in higher quality images, it becomes apparent that the “third arm” is not emanating from the core. From my perspective, a true arm should emanate from the inner region of the galaxy disk and not simply pop up in the other disk as a fragment.
In any case, I thought would be fun to put together the 3 arm galaxy poster that you see here, with the best 3 arm galaxy candidates I could find from the 1200+ galaxies that I have imaged. I am sure that there are more 3 arm candidates out there that I have missed.
The three categories of 3 arm galaxies listed in the poster are shown below, followed by the Astrobin link for each object. All of these objects are seen in my
Astrobin 3 Arm Galaxy Collection:
1. The Arp 3 arm galaxies (Arp 18-20) in Arp’s section of his catalog that he called, naturally, 3 Arm Galaxies:
Arp 19Arp 20Arp 212. In my opinion, better examples of 3-arm galaxies that I have found through my imaging:
NGC 7309NGC 1242NGC 613NGC 2835M833. Other examples of 3-arm galaxies that take more imagination to see:
NGC 7223NGC 7102NGC 1179NGC 1187NGC 5230NGC 6106M99NGC 2750NGC 6951NGC 7137NGC 7437NGC 7132What do you think – is the 3 arm galaxy a true structure or not?
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