Illustration of the disc’s DF F_⋆ from Eq. (8), in action space J = (L,I). It was assumed here that all stars are prograde, so that L> 0. Moreover, the action coordinates satisfy I ≥ L, so that I = L corresponds to circular orbits. The contours are spaced linearly between 95% and 5% of the DF maximum. The grey dashed lines correspond to the domain in action space from Eq. (14), to which the computations are restricted.