This is another of my continuing compilation poster series studying DSO features that I find interesting. The subject today is something that I refer to as Open Arm spiral galaxies.
Over the years I have seen an unusual number of spiral galaxies which seem to have cleared out space on either side of one arm, with a large visible gap on the trailing side. For this compilation poster, I have assembled 20 galaxies which I think are the best examples of this phenomenon. I have oriented all of the galaxies so that the open arm is heading up to the left, with the trailing gap on the right side of the open arm. Sometimes this involved flipping the galaxy so that it is a mirror image of our apparent view.
The galaxies share these common characteristics:
- Spiral galaxies, both barred and unbarred.
- Some evidence of VV rows, although this is inconsistent.
- Strong (distinct) arm structures.
- Core mostly remains well centered within the disk.
- Gap (behind open arm) consistently of around 45 degrees
- Ahead of the open arm, sometimes up to 180 degrees, there are no other arm structures.
- Few signs of interacting galaxies or companions.
The following questions come to mind:
- What is causing this open arm effect?
- Is this effect "permanent", lasting a billion years or so and several galaxy rotations, or is it transitory?
Disturbance by another passing galaxy or a companion would be the logical explanation, but my eyes don't see many obvious explanations in this regard for this collection. I believe that a common cause is behind the open arm effect and that the similarities in these galaxies are not just coincidence, but I just don't know what the common cause could be. Perhaps it is related to the VV row effect, which to this point is also poorly understood.
All of the above galaxies are posted and described individually on Astrobin and are contained in my
Astrobin Open Arm Spiral Collection.
If you enjoyed this compilation poster, please see my collection of 100 such posters in my
Deep Sky Compendium.
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