Top: size of area corresponding to flat radio spectral index (in kpc²) versus integrated radio spectral index calculated between 1.49 and 8.44 GHz. The dotted horizontal line marks the flat to steep division in radio spectral index. The Spearman ρ = −0.317 with 4% probability that the correlation is not present. Bottom: mid-IR area in kpc² of the core of mid-IR emission, assuming emission from a circular area (see Sect. 4.3), versus integrated radio spectral index (see Table 1). The Spearman ρ = −0.189 with 21% probability that the correlation is not present. In both plots, the dotted vertical line marks the flat to steep division in the radio spectral index. Symbols as in Fig. 3; unresolved objects are marked with circles instead of diamonds.