NGC 3184. Panel a: radio spectral index distribution between 142 and 1365 MHz, presented in a cube-helix colour-scale ranging from −1.2 to −0.4. Dashed contours are at −0.85 and −0.65, thus separating the galaxy in three, non-coherent areas. Areas with young CREs (α >  −0.65) are predominantly found in spiral arms; areas with CREs of intermediate age (−0.85 ≤ α ≤ −0.65) are predominantly found in inter-arm regions; and areas with old CREs (α <  −0.85) are found in the galaxy’s outskirts. Panel b: error of the radio spectral index distribution between 145 and 1365 MHz, presented in a cube-helix colour-scale ranging from 0 to 0.2. As can bee seen, only in areas of steep spectral indices α <  −0.85 the spectral index error becomes larger than ±0.1. In both panels, the maps were convolved to a circular synthesised beam of 18.6 × 18.6 arcsec² resolution, which is outlined in the bottom left corner. A mask has been applied to background sources and the central regions of NGC 3184. A 3σ cut-off was applied to both the 142- and 1365 MHz maps prior to combination.