Average starlight intensity ⟨ U ⟩ and σU for the DGS (crosses) and KINGFISH samples. For the KINGFISH sample, we distinguish the sources that have been classified as star forming (SF, downward triangles) or as AGN (stars) by Kennicutt et al. (2011). The average dust equilibrium temperature is shown on the top axis. The colour codes the sSFR of the sources, in units of Gyr^-1. We indicate the location of the two most metal-poor galaxies, I Zw 18 and SBS 0335-052, and of the three galaxies of Fig. 8.