Scatter plots of the non-thermal in-band spectral index distributions (6 GHz vs. 1.5 GHz; both are equal along the solid black line), in comparison to different spectral ageing models. a: NGC 891 (Fig. 6b vs. Fig. 6e). b: NGC 4565 (Fig. 7b vs. Fig. 7e). All α_nth maps were cut off at 30σ (NGC 891) or 20σ (NGC 4565) of the respective total intensity maps. Each data point represents the 1.5 and 6 GHz spectral index averaged within one synthesised beam and is colour-coded based on the 6 GHz total intensity map (Figs. 3b and 4b). The dash-dotted, dotted, and dashed lines show the expected positions in the plot for the CI, KP, and JP models, respectively, for different break frequencies ν_br and three different injection spectral indices α_inj. The symbols with error bars represent the Δα maps (panels c and f of Figs. 6 and 7) and are shown for three different intensity values.