The fraction of particles on retrograde orbits after the fly-by interaction of two equal-mass galaxies. They have apocentres above 30 kpc (P_retro, upper panel), and above 60 kpc (, lower panel). See Tables B.1 and B.2 for the values. The fraction of retrograde material is well defined by the initial velocity only, without much dependence on the minimum distance r_min at which the two galaxies pass each other. P_retro increases with increasing initial velocity, which ranges from 1.8 (“180vel”) to 2.4 (“240vel”) times the the parabolic velocity. See the on-line edition of the article for a colour version of this figure.