Histogram of the Euclidean norm from each radio source to its closest representative image in the final 10  ×  10 SOM, for each of the 19 544 radio sources in our final training set. Most sources have a very small Euclidean norm to their best resembling representative image in the SOM, which means they are morphologically similar to at least one representative image in the SOM. We isolated the 100 most outlying sources and show them in Appendix B.