Upper panel: observed total infrared luminosity as a function of the model HCO⁺(1–0) luminosity. The symbols are the same as in Fig. 10. The dashed line represents the relation found by Juneau et al. (2009). The dotted line represents the relation found by Garcia-Burillo et al. (2012) with L_TIR = 1.28 × L_FIR. Lower panel: model HCO⁺ (1–0) luminosity as a function of the observed HCO⁺(1–0) luminosity. The solid line corresponds to equality, the dotted lines to factors of 1/2 and 2. The dashed line represents a robust bisector fit to the data.