Radio spectral index (i.e. ) of galaxies as a function of Δlog (SSFR)_MS, as derived from our stacking analysis. Hatched areas represent the regions of parameter space affected by incompleteness (see text and Fig. 3). In each panel, we give the median value (see also the green lines), the Spearman rank correlation (ρ_s) and the null hypothesis probability (Sig.) derived from data points in the region of parameter space not affected by incompleteness. Red dashed lines correspond to the canonical radio spectral index of 0.8 observed in local star-forming galaxies (Condon 1992) and high-redshift SMGs (Ibar et al. 2010). Shaded regions show the range of values (i.e. 0.7 ± 0.4) observed by Ibar et al. (2009) in a population of sub-mJy radio galaxies. Vertical solid and dot-dashed lines show the localisation and width of the MS of star formation. In the range of redshift and Δlog (SSFR)_MS probed here, does not significantly deviate from its canonical value, 0.8.