Apocentre distances for merger model 7.5deg100vel (left) and fly-by model 5deg200vel (right). Plotted are the numbers of particles in bins of d_apo with width 4.0 kpc, starting at 30 kpc. In case of a merger, particles on pro- and retrograde orbits (within the respective COAs ) can be found at all apocentre distances from 30 up to about 140 kpc. In the fly-by case, in contrast, pro- and retrograde particles have distinct apocentre properties. The retrograde particles have lower apocentres (up to about 80 kpc) while the prograde particles have maximum apocentre distances of 120 kpc, but only few of them are found with d_apo below 50 kpc. This hints at the two-phase origin typical for fly-by interactions. Particles with retrograde orbits are formed first, while the passing galaxies are still close to each other, and thus have lower average apocentre distances.