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Fig. 4

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Spectra around a) the OH84 doublet, and b) the OH65 doublet, in Mrk 231 (upper spectra), Arp 220 (middle), and NGC 4418 (lower). The vertical-dotted lines in a) indicate the positions of NH3 and 18OH lines. The NH3 lines at v < −2000 km s-1 are relatively strong in NGC 4418, indicating contribution by NH3 to the absorption at v ~ 1000 km s-1. This is not the case of Mrk 231, so that there is no significant contamination at v ~ 1000 km s-1. The 18OH66 doublet in b) is complex with probable contributions by o-H2O+ and NH2.

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