Linearised (Σ_SFR)_RC–(Σ_SFR)_hyb relation after convolving the (Σ_SFR)_hyb map with a Gaussian kernel to simulate the diffusion of CREs. Left panels: results for the 140 MHz LOFAR data; right panels: results for the 1365 MHz WSRT data. The length of the Gaussian kernel, l (=FWHM/2), is noted as well. Panels a and b: results for NGC 3184, panels c and d: for NGC 4736, and panels e and f: for NGC 5194. Each data point represents a 1.2 × 1.2 kpc² regions that has been obtained once from the hybrid Σ_SFR map (abscissa) and once from the radio Σ_SFR map (ordinate). Shape and colour represent different radio spectral indices between 145 and 1365 MHz. Downward-pointing red triangles represent regions with young CREs (−0.65 <  α <  −0.20); filled green circles represent regions with CREs of intermediate age (−0.85 ≤ α ≤ −0.65); and upward-pointing blue triangles represent regions with old CREs (α <  −0.85). Solid black lines show the best-fitting relation and dashed lines show the Condon relation. A 3σ cut-off was applied in all maps.