A larger galaxy may have a smaller galaxy orbiting around it called a satellite galaxy. This article explores the process of the smaller galaxy orbiting the larger one, with a focus on how the satellite galaxy might encounter friction from the halo surrounding the larger galaxy, leading to a decaying orbit. This friction eventually causes the satellite galaxy to spiral inward and merge with the larger galaxy.

http://www.raa-journal.org/raa/index.php/raa/article/view/510