Spectral index distributions. a) Comparison of the spectral index distribution for the high-RM candidates (30 sources; red histogram) and the unpolarised sources (77 sources; black histogram). The distribution of the unpolarised sources peaks at α_peak ~ −0.5. The high-RM distribution shows 3 different types of objects: steep (α_peak ~ −0.8), flat (α_peak ~ −0.1), and inverted (α_peak ~ + 0.2) spectrum radio sources. b) The same but cumulative histograms.