The absorption or accretion of smaller galaxies into larger ones is called what

The absorption or accretion of smaller galaxies into larger ones is called _________ __________ .
I read in the book that when two smaller galaxies collide into one it is called Cartwheel Galaxy but the question marked me wrong. Can you please help me out?

Thank you

There are any number of terms to describe the event in your question --- galactic cannibalism, galactic collision, etc.,.

The Cartwheel Galaxy is just one galaxy that was formed by the merger of 2 smaller galaxies.

Its not the name for all of them - the term is "merger".

http://en.wikipedia.org/wiki/Cartwheel_g…