Below is a link to a nasa.gov article regarding NGC922 aka the Bullseye galaxy and a recently released image from the Hubble space telescope. It has been described as rare due to that it has 9 rings surrounding it. It has also been said that the reason for these 9 rings is that a smaller galaxy collided into it through it's centre, the remnants of this smaller galaxy can be seen in the image.

Do any members of SF that are interested in this field know of the mechanism of how such a collision would cause these 9 rings?

 

https://science.nasa.gov/missions/hubble/hubble-investigates-galaxy-with-nine-rings/

An image of the Bullseye from the above link, credit to NASA and Hubble