This is another compilation poster in my continuing series studying DSO features that I find to be interesting.
The subject today is Pseudo-Ring Galaxies. Although this is the formal name for this feature, a more descriptive term is probably “S” Galaxies, since most of them strongly resemble an “S”. They also have been called Figure 8 Galaxies.
Galaxy rings are quite common and take on different forms. My poster on galaxy rings is
here.
When most people think of galaxy rings, they often think of closed outer rings. But closed rings are actually quite rare, because they typically occur in older galaxies with no tidal disturbance. You can see them in my
Astrobin Closed Outer Ring Collection.Pseudo-rings are much more common than closed rings and are basically tightly wound spiral arms. As seen in the poster, most galaxies with pseudo-rings have a inner oval lens with a bar. Arms extend from each end of the bar, emanating for 180 degrees around the inner lens. The end of the arm terminates just outside of the opposite end of the bar, sometimes integrating almost seamlessly into the opposite arm.
For consistency on the poster, I have flipped some of the galaxies so that all appear to have arms extending in the clockwise direction. It was easier for me to flip them than to take an image from the opposite side of the galaxy.
A few comments on these rings:
- Studies indicate that closed outer rings are a more evolved stage occurring in early type barred galaxies (like SB0), while pseudo-rings occur in later types.
- No nearby companions are seen in the poster, and indeed it is believed that tidal disturbance by a companion destroys a ring.
- Simulations do a nice job of modeling the formation of these rings over time (see figure below). Simulations also show ring breakdown with companion tidal interaction.
- Galaxies with pseudo-rings but no visible bar are believed to have had a bar in the past.
All of the above galaxies are posted and described individually on Astrobin and are contained in my
Astrobin Pseudo-Ring Galaxy Collection.
If you enjoyed this compilation poster, please see my collection of 100 such posters in my
Deep Sky Compendium.
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Example simulation of undisturbed galaxy from the 1992 paper by Elmegreen et al,
"The Influence of Environment on Outer Rings and Pseudo-Rings in Galaxies":
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