The ratio of size of the area corresponding to flat radio spectral index (in kpc²) as in Table 5, to the total radio size as in the α-maps versus median radio spectral index calculated from the α-maps. The dispersion (standard deviation) around the median is shown with horizontal dotter error-bars. Symbols: blue open diamonds denote the objects of our sample, blue filled diamonds denote objects that are classified as radio-AGN based on the α-maps, and black squares denote objects that have an AGN contribution in their mid-IR luminosity (Petric et al. 2011); unresolved objects are marked with circles instead of diamonds. The vertical dotted lines at 0.5 and 1.1 mark the flat-steep division and the limit above where we do not expect steepening from compact starburst, respectively.