Notes. b and v_rel are the impact parameter and the relative velocity between the centres of mass (CMs) of the two galaxies at the initial distance, respectively. For the definition of θ, φ, ψ, see Fig. 1 of Hut & Bahcall (1983). In particular, θ is the angle between the relative velocity vector v_rel and the symmetry axis of the primary disc, φ describes the orientation of v_rel projected in the plane of the primary disc and ψ describes the orientation of the initial distance vector D (between the CMs of the two galaxies) in the plane perpendicular to v_rel. E_s is the specific orbital energy, i.e. the total energy divided by the reduced mass μ = m₁m₂/ (m₁ + m₂), where m₁ and m₂ are the mass of the primary and of the secondary galaxy, respectively. , where M = m₁ + m₂ is the total mass of the two galaxies, G is the gravitational constant and D the initial distance between the CMs. L_s is the modulus of the specific orbital angular momentum, i.e. the angular momentum divided by the reduced mass. e is the eccentricity (). A orbit is classified as prograde/retrograde depending on the alignment/counter-alignment of the orbital angular momentum of the secondary galaxy with respect to the spin of the primary galaxy.