Time evolution of a galactic disc with a BH at rest at the centre of coordinates. The black dot indicates the BH of mass =1.25 × 10⁷ M_⊙. A white dot indicates a test star which remains on a steady orbit. The time runs from left to right as indicated. Notice how an over-density in the shape of a ring develops early on but is then wiped out by differential rotation. After ≃1 Gyr of integration time, the BH remains close to the barycentre with a velocity of ∼7km s¹ in amplitude.