Spectral index maps of the five galaxies with extended radio emission. Dashed red circles mark unrelated sources that were masked. The maps were created using pixels above a 3σ significance in both the 144 MHz LOFAR and 1.28 GHz MeerKAT measurements at the same circular synthesized beam size of 20″, as is indicated by the circle in the lower left corner. The colour scales are different in each image. Upper limits of the spectral index are indicated by downward-pointing arrows. For the low-surface-brightness emission of NGC 4472, we instead used the LOFAR compact source-masked and the MeerKAT compact source-subtracted images at a resolution of 1′ and display pixels above a statistical significance of 2.5σ in both images. (a) NGC4374. (b) NGC4472. (c) NGC4486. (d) NGC4552. (e) NGC4649.