The collisional ring theory, developed by Arp and others, is that these objects are created when a larger galaxy experiences a direct a head-on collision, along its polar axis, with a smaller galaxy. The collision creates an expanding density wave of star formation, resulting in a ring in the larger galaxy. The smaller galaxy is usually significantly disturbed after the collision.

At first, I found such a theory hard to believe – what are the chances of 2 galaxies being in such an exact alignment for such a collision? But indeed, examining the poster shows numerous convincing examples of such an interaction. Notice in some of the objects how the rim of the ring becomes bright, and often blue, due to the resulting star formation. Also, note that remnants of the smaller galaxy are often seen nearby the ring, although in some of these cases I believe that the smaller galaxies are more distant and not necessarily related to the event.

The numbers in the poster show the distance away from us for each object in billions of light years. Also, the objects in the poster are sized approximately to scale. Many of the objects are close to 100,000 light years in diameter. I find it interesting that their sizes are so consistent. Perhaps smaller galaxies are torn apart by the collision, while larger galaxies absorb it without the dramatic ring effect.

I have only included objects in this poster which I have imaged and which seem to me to be likely examples of such a collision. A literature search will find a number of papers which list hundreds of collisional ring galaxy examples. But, in my experience imaging some of these other objects, I am not convinced that all of them are true collisional galaxies. Many simply look like star stream loops, which happen in peculiar galaxies with surprising frequency. Loops will be the subject of an upcoming poster.

An example of an object for which I have some question is Arp 219, shown at upper right in the poster. Note the multiple loops and the extended star stream down and to the left. Many of the most convincing collisional ring examples I have seen have neither multiple loops or extended star streams. But perhaps the hit of Arp 219 was off center, which I think could lead to unpredictable results.

If you would like to read more about any of these objects, each of the objects in the poster has previously been uploaded and described individually on Astrobin. They all reside in my Astrobin Collisional Ring Collection.

If you would like to access all my DSO compilation posters, please click here.