H2O megamasers: Accretion disks, jet interaction,outflows or massive star formation?

The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H20 emission. Single-dish monitoring and high-resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1–3 pc were mapped and masses of nuclear engines of order 106–108 Mʘ were determined. So far, ∼50 extragalactic H20 maser sources have been detected, but few have been studied in detail.