Illustration of the active surface density Σ_t for a N-body run with N = 8 × 10⁵, restriced to the range R ≤ 6. Top panel: active surface density Σ_t at an early time t = 60, for which the galaxy remains globally axisymmetric. In this regime, the dynamics of the system is collisional and governed by the Balescu-Lenard Eq. (2). Bottom panel: active surface density Σ_t at a much later time t = 2400. The galaxy is then strongly non-axisymmetric. In this regime, the dynamics of the system is collisionless and governed by the Vlasov equation.