WISE colour–colour diagram showing the five subsamples, as indicated by the symbols. The uncertainties are comparable to the symbol sizes. The large symbols with error bars correspond to the mean values for each sub-sample, with the error bars representing the standard deviation. The dashed line represents the threshold W1 − W2 = 0.8, while the dotted polygon marks the region typically occupied by Seyfert galaxies (Jarrett et al. 2011).