Continuum-normalized spectra around the OH⁺ 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. (Potential) contamination by other species (NH in panel a) and H₂O in d)) is also indicated. The marginal 5_J − 4_J′ lines in f) correspond to NGC 4418; this part of the spectrum is not available in Arp 220. Blue lines show model fits for OH⁺ in the nuclear components of both NGC 4418 and Arp 220, the green lines show the model fit for OH⁺ in the extended components of both sources, and the light-blue lines in a) are model fits for NH; red is total.