OH Rotational Lines as a Diagnostic of the Warm Neutral Gas in Galaxies

Abstract

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 (Tk ~ 150 K, n 5 × 104 cm-3) 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 (~5 × 105 cm-3) are required to explain the OH emission in NGC 1068.