Radio spectral index as a function of several fundamental galaxy parameters. Panel a: dependence on SFR as calculated from the TIR luminosity; (b): dependence of the star formation radius, which is derived from the extent of the 3σ contours in the 20 arcsec LoTSS images; (c): dependence on the total mass calculated; (d): dependence on the rotation speed. In all panels, the blue line is the best-fitting relation and the shaded areas show 1σ confidence intervals. The colours of the data points in panel b indicate the rotation velocity, in the other panels the colours indicate the different subsamples.