HCN line fluxes in NGC 4418 a) and Arp 220 b). In Arp 220, the fluxes of lines with E_lower up to 570 K have been taken from SPIRE-FTS data (Rangwala et al. 2011). The curves and triangles show model predictions for the C_core component of NGC 4418 and the C_west component of Arp 220. The dotted curve in b) shows results obtained when only collisional excitation is included in the model. The solid curve in a) shows the best fit model obtained with τ₂₀₀ ≈ 1, while the dashed and dotted curves correspond to τ₂₀₀ = 4 with and without gas-dust mixing, respectively.