Top row: observed V distribution for Gaia DR2 stars inside spherical volumes (of radius 0.2 kpc) centred at different Galactocentric radii and along the line connecting the Galactic centre to the Sun; the distances of the stars used here are those provided by Bailer-Jones et al. (2018). Bottom row: v distribution obtained from the backward integration already shown in Fig. 7 and marginalizing along the u-axis (assuming V_⊙ = 0). The red lines correspond to the same velocities in both rows, and to the same angular momentum as that of the gap at R = 8.2 kpc.