Dependency of three important parameters on the temperature of the H ₂ gas, calculated from the S(2) and S(1) lines. Top: the relative strength of the PAH emission (given by the flux sum of all PAH features between 10 and 20 μm normalized to the continuum flux at 15 μm). Center: the hardness of the radiation field (as given by the line flux ratio of [Ne iii]/[Ne ii]). One outlier, the nucleus of NGC 4038, which shows a very low [Ne iii]/[Ne ii] ratio, is not included. Bottom: the SFR as listed in Table 9. The dotted lines are linear fits but only intended to guide the eye.