Top and middle panels: large-scale source structure of FIR 4 (n_gas at top, T_gas at middle) from Crimier et al. (2009, C09), corresponding to an integrated luminosity of 1000 L_⊙. Bottom panel: relative populations of the lowest HCl rotational states, showing J = 0 (black), J = 1 (blue), and J = 2 (red) for the C09 source structure without modification (solid lines) and with an added external irradiation of G₀ = 415 ISRF (dashed lines; Sect. 5.4). The level populations were modelled with Ratran using the new hyperfine collision rate coefficients presented in Appendix A.