Upper panel: observed total infrared luminosity as a function of the model HCN(1–0) luminosity. The symbols are the same as in Fig. 10. In addition, orange stars represent the observed HCN(1–0) of an ULIRG subsample (from Graciá-Carpio et al. 2008). The solid line represents the correlation found by Gao & Solomon (2004). The dashed line represents the correlation found by Graciá-Carpio et al. (2008) with L_TIR = 1.28 × L_FIR: . Lower panel shows the model HCN(1–0) luminosity as a function of the observed HCN(1–0) luminosity (Graciá-Carpio et al. 2008) for individual galaxies. The dashed line corresponds to a robust bisector fit.