Optical depth of Compton thick sources normalized at 9.7 μm. Left: observed profiles of the optical depth profiles of NGC 4418 (brown line), NGC 5728 (purple line), and ESO 137-G034 (black solid line). Right: zoomed-in image of the H₂O bending mode absorption band. Laboratory spectra of pure water (blue shaded region corresponds to H₂O at 15 K; Ehrenfreund et al. 1997; Öberg et al. 2007) and an a:C–H hydrogenated amorphous carbon analog (red shaded region; Dartois & Muñoz-Caro 2007, see also Maté et al. 2019) are shown (see Appendix C for further details of the laboratory spectra).