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

We present Infrared Space Observatory (ISO) observations of several OH, CHand H2O rotational lines toward the bright infrared galaxies NGC253 andNGC1068. As found in the Galactic clouds in SgrB2 and Orion, the extragalacticfar-IR OH lines change from absorption to emission depending on the physicalconditions and distribution of gas and dust along the line of sight. As aresult, most of the OH rotational lines that appear in absorption toward NGC253are observed in emission toward NGC1068. We show that the far-IR spectrum of OHcan be used as a powerful diagnostic to derive the physical conditions ofextragalactic neutral gas. In particular, we find that a warm (Tk~150 K, n(H2)<5 10^4 cm^-3) component of molecular gas with an OH abundance of 10^{-7} fromthe inner <15'' can qualitatively reproduce the OH lines toward NGC253. Similartemperatures but higher densities (5 10^5 cm^-3) are required to explain the OHemission in NGC1068.Comment: 5 pages, 4 figures, accepted in ApJ Part I (2004, October 6