Plot of the combined data, showing the spatially resolved RC–SFR ((Σ_SFR)_RC–(Σ_SFR)_hyb) relation. Panel a: results from LOFAR 140 MHz and panel b: from WSRT 1365 MHz. Each data point represents a 1.2 × 1.2 kpc² region that has been obtained from the hybrid Σ_SFR map (abscissa) and from the radio Σ_SFR map (ordinate). Shape and colour represent different radio spectral indices between 140 and 1365 MHz. Downward-pointing red triangles represent regions with young CREs (−0.65 <  α <  −0.50); 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.