Hydroxyl absorption toward galactic nuclei

Abstract

We report the detection of hydroxyl (OH) absorption toward the nuclear continuum sources in NGC 660, NGC 3227, NGC 3504, NGC 3628, and NGC 5363. The peak apparent optical depths are typically a few percent. The line profiles are unpolarized, and the OH main lines are in roughly their optically thin LTE intensity ratios. The line widths are particularly large for NGC 660 and NGC 3628, with structure reminiscent of the OH profiles seen in the nucleus of our Galaxy. We argue that the absorption arises mostly in molecular disks that are within a few hundred parsecs of the nuclei. A comparison with negative results suggests that such molecular sources are common in late-type galaxies, but perhaps not universal.