2018ApJ...853..132S

We analyze Spitzer/InfraRed Spectrograph (IRS) observations of the OH 35 µm feature in 15 nearby (z   0.06) (ultra-)luminous infrared galaxies (U/LIRGs). All objects exhibit OH 35 µm purely in absorption, as expected. The small optical depth of this transition makes the strength of this feature a good indicator of the true OH column density. The measured OH 35 µm equivalent widths imply an average OH column density and a 1-σ standard deviation to the mean of N_OH=1.31±0.22×10¹⁷ cm^–2. This number is then compared with the hydrogen column density for a typical optical depth at 35 µm of ∼0.5 and gas-to-dust ratio of 125 to derive an OH-to-H abundance ratio of X_OH=1.01±0.15×10^–6. This abundance ratio is formally a lower limit. It is consistent with the values generally assumed in the literature. The OH 35 µm line profiles predicted from published radiative transfer models constrained by observations of OH 65, 79, 84, and 119 µm in 5 objects (Mrk 231, Mrk 273, IRAS F05189-2524, IRAS F08572+3915, and IRAS F20551-4250) are also found to be consistent with the IRS OH 35 µm spectra.