Computing the column density profile for HFSs: the column density map of each target is masked by the skeleton (left panel) and then averaged for all target hubs (middle panel) and non-hubs (right panel). The radial profile shown in Fig. 11 is computed using these average images for different evolutionary groups. The example shown here is for the group of proto-stellar hubs and non-hubs, and the units of the colour bar are cm⁻².