OH lines in NGC 4418 (upper spectra) and Arp 220 (lower spectra). Λ-doubling is resolved in most transitions, except for the two narrowly separated 71 μm components that are blended into a single spectral feature. Model results are also shown. For NGC 4418, the blue, light-blue, and green curves show the models for the C_core , C_warm , and C_extended components, respectively; for Arp 220, the blue, light-blue, green, and yellow curves show the models for C_west , C_east , C_extended , and C_halo , respectively. The gray curves in panel f) show the expected contribution of the H₂O 6₂₅ → 5₁₄ line to the spectra around 65.2 μm. The red curves show the total from all components.