Continuum-normalized spectra around the H₂O⁺ lines in NGC 4418 (upper histograms) and Arp 220 (lower histograms). The dotted vertical lines indicate the rest wavelengths of the transitions for the nominal redshifts of z = 0.00705 (NGC 4418) and 0.0181 (Arp 220). The spectra in panels b) and c) have been scaled by a factor 2. Close features due to other species and potential contaminations are indicated. Dark blue and light blue lines show model fits; red is total.