2005ApJ...619..291G

We present Infrared Space Observatory (ISO) observations of several OH, CH, and H₂O rotational lines toward the bright infrared galaxies NGC 253 and NGC 1068. As found in the Galactic clouds in Sgr B2 and Orion, the extragalactic far-IR OH lines change from absorption to emission depending on the physical conditions and distribution of gas and dust along the line of sight. As a result, most of the OH rotational lines that appear in absorption toward NGC 253 are observed in emission toward NGC 1068. We show that the far-IR spectrum of OH can be used as a powerful diagnostic to derive the physical conditions of extragalactic neutral gas. In particular, we find that a warm (T_k∼150 K, n_H2≲5x10⁴/cm3) component of molecular gas with an OH abundance of ≃10^–7 from the inner ≲15" can qualitatively reproduce the OH lines toward NGC 253. Similar temperatures but higher densities (∼5x10⁵/cm3) are required to explain the OH emission in NGC 1068.