Radial column density profile for G28.37+0.07, G28.53-0.25, and G18.82-0.28. Each cross in the panels represents one pixel value in the map. The x-axis gives the distance to the centre of the ellipse (x-axis). The dashed orange line indicates the Herschel beam (Gaussian profile with a FWHM of 36′′ displayed in log-log format). The exponent α was deduced from fitting the column density profile (N ∝ r^1−α). For the fit, we included only the pixels that were inside a radius of 3 pc.