Each row corresponds to an object from our sample. Left: radio-spectral-index maps (α-maps) are represented with the colour-map; the colour bar on the right shows the radio spectral index values from low (flat α) to high values (steep α). Overlaid, the radio contours at 1.49 GHz (black dashed) and at 8.44 GHz (white solid), where the contour levels scale as log₁₀ of the flux density from the lowest flux value (3σ) to the highest. Middle: the error map for the radio spectral index per pixel value. The colour bar on the right shows the range of values. Contours as on the Left. In both plots, on the bottom left corner we give the beam of the 1.49 GHz map as black circle, and the beam of the 8.44 GHz map as grey circle/ellipse. On the bottom right we give a scale-bar of 2 arcsec, as visual aid. Right: histograms of the α-maps: Top: histograms of the radio spectral index, based on Fig. 2-Left, that give the values of α at each pixel. Middle: histograms where the errors on α (Fig. 2-middle) have been added. Bottom: histograms where the errors on α have been subtracted. The rest of the figure is located in the Appendix.