Optical depth of the Compton thick sources normalized at 9.7 μm. Left: 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. The laboratory spectra of pure water (blue and green shaded regions correspond to H₂O at 15 and 115 K, respectively; Ehrenfreund et al. 1997; Öberg et al. 2007) and a a:C-H hydrogenated amorphous carbon analog (red shaded region; Dartois & Muñoz-Caro 2007, see also Maté et al. 2019) are shown (see Spoon et al. 2022, their Fig. 29, for a similar plot).