Pixel-to-pixel correlation between the PACS cooling lines ([C ii], [O i]₆₃, and their sum) and 2 quantities used to trace the gas heating, i.e., the TIR emission (left panel) and the PAH emission (right panel). The solid curve represents the linear regression for a given plot and the dashed curves the linear regression of the other ratios for comparison. The colors correspond to specific regions illustrated in Fig. 12. The symbol size is proportional to [C ii]+[O i]₆₃ (top), [C ii] (middle), and [O i]₆₃ (bottom). The gray rectangles on the left side show the values observed by Croxall et al. (2012) in 2 super-solar metallicity nearby galaxies (see text).