Maximal velocity of colliding galaxies

In the current galaxy formation paradigm, collisions play a crucial role. A fraction of galaxy collisions results in fly-by events, and a galaxy that has passed through another galaxy is called a backsplash galaxy. Such fly-by events are of particular interest for explaining the quenching of isolated galaxies. One signature of backsplash galaxies is that they have high velocities relative to their environment, since they do not move in the same flow as surrounding galaxies. This feature can be studied in simulations, but it is also useful to have a theory that can predict the velocities of backsplash galaxies. In this paper, we develop such a theory based on the Zel'dovich approximation and use it to determine the maximal expected velocity of a backsplash galaxy in a given volume.