Normalized distributions of the dust-mass fraction heated by PDRs (γ; top panel), the constant intensity of the radiation field (U_min; middle) and the polycyclic aromatic hydrocarbon index (q_PAH; bottom) of the DL07 templates used in our sample. Histograms show the distribution of the full sample (black), SB (blue), MS (red), and PAS (green) galaxies. In the bottom panel the q_pah histogram is divided in three groups of values, shown by the purple solid and black dashed lines: Milky Way models (left side of the solid line), the Large Magellanic Cloud (between purple solid and black dashed lines), and the Small Magellanic Cloud (right side of the black dashed line).