I was just looking at
this image (NGC 4314), and I noticed the interesting ring of glowing nebulae about half way from the center of the galaxy. This got me thinking, "ring galaxy", not that I meant this galaxy was one, but it got me thinking of ring galaxies. Then I noticed the bright core, and I started thinking that maybe if you left out the dust and gas between the core and the ring of star formation... Maybe you get a ring galaxy. To me it seems really easy to take out the "little" amount of stars outside and inside the ring, besides the core...
So, maybe this is how they form...
1. A normal spiral galaxy forms.
2. Something happens and a ring of nebulae and star formation occurs (now this is the mystery!)
3. As the galaxy ages, the stars outside the ring and inside die out, and little star formation occurs.
4. The dust scatters.
5. Between step 2 and 4, the ring of formation has gone through some generations and create more stars to keep it replenished.
This is my theory/hypothesis on how ring galaxies are formed.