Now, im not that good with physics and gravity, but my theory is that 2 galaxies collide, but the super massive black holes in the center dont merge, they just spin around each other millions of times per second, making the orbit off the stars of the galaxy much shorter, and faster, causing them to fly out ward until they stop, in a ring. My other theory is, for some reason (perhaps a huge gravitational event Ex. getting close to another galaxy) all the stars in the galaxy simply fly away until they stop at some gravitational border, where its close enough the stars dont whip off into the universe, and far enough that they dont get sucked into the super black hole in the center. i have know understanding of how a ring without a center could exist, Ex. if you took away saturn, it's rings could not exist.