Constraints on the OH-to-H Abundance Ratio in Infrared-bright Galaxies Derived from the Strength of the OH 35 μm Absorption Feature

Abstract

We analyze Spitzer/ InfraRed Spectrograph (IRS) observations of the OH 35 μ m feature in 15 nearby ( ) (ultra-)luminous infrared galaxies (U/LIRGs). All objects exhibit OH 35 μ m 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 μ m equivalent widths imply an average OH column density and a 1- σ standard deviation to the mean of cm −2 . This number is then compared with the hydrogen column density for a typical optical depth at 35 μ m of ∼0.5 and gas-to-dust ratio of 125 to derive an OH-to-H abundance ratio of . This abundance ratio is formally a lower limit. It is consistent with the values generally assumed in the literature. The OH 35 μ m line profiles predicted from published radiative transfer models constrained by observations of OH 65, 79, 84, and 119 μ m 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 μ m spectra.