Top row: the J = 1 − 0 HCN/CO ratio as a function of IRAS FIR luminosity (left) and J = 1 − 0 CO luminosity (right). The horizontal line in the panel at left marks the ratio separating normal and luminous galaxies in the paper by Gao & Solomon (2004b). Neither panel shows any increase in HCN/CO at (U)LIRG luminosities, nor any change in the dispersion of the ratio. Center row: the HCN/CO ratio as a function of L(HCN), (left) and the FIR/CO ratio as a function of L(FIR), (right). Bottom row: the FIR/CO ratio versus HCN/CO ratio (left); the FIR/HCN ratio versus the FIR/CO ratio (right). In both panels, straight lines mark unity slopes. Fit parameters are listed in Table 8.