This poster is the 45th in my series of compilation posters.  All of these posters can be seen in my Astrobin Poster Collection.

This poster is a collection of galaxy rings.   I have struggled over the years trying to understand the characteristics and formation mechanisms of these wonderful objects.   Through a series of these posters, I am attempting to document and share what I have learned so far.  Some of the key astronomers in the field of ring galaxies include Buta, Kormendy, Schwarz, Athanassoula, Combes, Sellwood, and Elmegreen. 

About 20% of spiral galaxies contain rings and about 20% more contain partial rings.  Rings are always associated with the presence of gas and are the site of more intense star formation.  

Rings are almost always found in barred galaxies.  Bars are believed to be critical to the process of ring formation.  Some non-barred galaxies have rings, but it is believed that in those cases the original bar has dissipated.   Rings usually have a specific orientation relative to the bar, either parallel or perpendicular.  

For some reason, I used to think that most rings are the result of interactions or mergers with other galaxies.  This is not true.  Most rings are believed to be the result of simple resonance due to the actions of the bar on the gas clouds in the disk.  It takes a long time for rings to form - ring formation is described as a secular (slow) internal process.

Rings do not like the gravitational presence of other galaxies.  Galaxies with inner rings typically do not have close companions.  Simulations have shown that tidal interactions from nearby galaxies disrupt and destroy the ring formation process.  

This poster establishes ring definitions and provides a few examples.  The shape and angle of these rings depends on our viewing perspective.  I show 5 types of rings:

1.  A closed outer ring (R):
A closed outer ring is usually twice the size of the bar. Closed means looking like a "O", with a void area along the circumference of the inner and outer edges.  Closed outer rings are recognized with the symbol type (R). Rare cases of double outer ring galaxies (type RR) occur, where two detached outer rings are seen.

2.  An outer pseudoring (R1’) made of spiral arms, perpendicular to bar:
Pseudorings are much more common than closed rings.  Pseudorings are oval and look like tightly wound spiral arms, forming a rough and not a pure ring.  The oval orientation is perpendicular to the bar.   Like closed rings, this is a large, diffuse structure, about twice the size of the bar.

3.  An outer pseudoring (R2’) made of spiral arms, parallel to bar:
This is just like the pseudoring of type 2, except that the oval is oriented parallel to the bar.

4.  An inner ring (r), enveloping each end of a bar:
This is by far the most commonly seen galaxy ring.  The ring may be circular, oval, or even pointed at each end.  Extensive star formation is usually occurring in the ring and sometimes can be seen by a vivid blue color.

5.  A nuclear ring (nr) immediately surrounding the core.
This is the smallest and brightest ring feature.  Nuclear rings are about 1/10 the size of the bar.  The most intense star formation occurs in these types of rings.

Many of the objects in the poster contain more than one of these features.  For example, M95 contains a nuclear ring, an inner ring, and a pseudo-ring.

If you would like to read more about any of these objects, each of the objects in this poster has previously been uploaded and described individually on Astrobin. They all reside in these 5 Astrobin collections:

GALAXIES - Ring (Outer Closed)
GALAXIES - Ring (Pseudo-R1')
GALAXIES - Ring (Pseudo-R2')
GALAXIES - Ring (Inner)
GALAXIES - Ring (Nuclear)

The collections contain many more galaxies which did not make the cut for this poster.  Looking through these 5 collections, you will see that almost all of the 130+ galaxies contained therein are isolated and barred.