Understanding Extragalactic Hydroxyl

Hydroxyl (OH) is one of the few molecules in space showing both maser emission and absorption. In the Milky Way, hydroxyl has been found in various environments such as stellar envelopes, star-forming regions, and HII regions, tracing the physical states of the interstellar medium. Extragalactic OH has been found to provide essential information about the dusty and obscuring material within the inner kiloparsec region of active galaxies. At angular resolution of a few tens of parsecs, OH shows a rather complicated spatial distribution and the interpretation of the circumnuclear environment is rather difficult. Based on results obtained from low- and high-resolution observations, the diagnostic tools of OH will be reviewed and the strategies to further investigate extragalactic OH will be discussed.