Far-infrared colour–colour diagrams, modified from Kewley et al. (2000). Top: log (f_{60 μm}/f_{25 μm}) vs. log (f_{25 μm}/f_{12 μm}) diagram. Bottom: log (f_{60 μm}/f_{12 μm}) vs. log (f_{100 μm}/f_{25 μm}) diagram. In both plots, the solid line is the reddening line calculated by Dopita et al. (1998). The straight dashed line marks the typical area of starbursts. The “extreme mixing line” corresponds to the mixture of a typical Seyfert-1 spectrum and a starburst component (with denoted percentage). The open symbols are Seyfert-1, starburst, and Seyfert-2 galaxies with (HBLR) and without (Sy2) broad-line components observed in polarised light, taken from the sample of Rush et al. (1993). The positions of the analysed galaxies are indicated by the coloured data points.