Same as Fig. 2 but for the PAH–IR luminosity (left) and CO–IR luminosity relationships (right). The total IR luminosity (L(IR)) is calculated from 8 to 1000 μm from the best-fit SEDs (Fig. 1). Unlike the CO-PAH relation in Fig. 2, the PAH–IR and CO–IR luminosity ratios are not constant across the population – they decrease at high IR luminosities (ULIRG regime), leading to sub-linear fits.