Upper left panel: 2D projection on the plane (M_cl, r_hm) of the 1500 clusters integrated with CMC. Clusters in which no merging BBH are formed are indicated as grey dots, while the coloured dots represent the number of merging BBHs formed in each cluster. The diagonal dashed grey lines represent lines of constant density ${\rho }_{\mathrm{c}\mathrm{l}}\equiv M_{\mathrm{c}\mathrm{l}}/(4/3)\pi r_{\mathrm{h}\mathrm{m}}^3$. Upper right panel: 2D projection on the plane (M_cl, Z_cl). Colours indicate the number of merging BBHs with the same norm as in the left-hand panel. Lower panel: 3D grid in parameter space (M_cl, r_hm, Z_cl).