I got it from "James Binney and Scott Tremaine (2008). Galactic Dynamics: Second Edition" (section 8.2
High-speed encounters, page 655). My question is about page 657, between equation (8.29) and (8.30).
So consider a subject system, a galaxy, with stars in it. This galaxy is passed by another object, the perturber. When the interaction is fast, we call this interaction a high-speed encounter.
Now we consider that the interaction is really fast. This means that during the interaction, the...