A "clear" view of the nucleus: the Megamaser perspective

Extragalactic emission from the hydroxyl and the water molecule was first detected in the early eighties, revealing a new class of maser emission with unexpected isotropic luminosities of many magnitudes higher than their galactic counterparts. Galaxies that harbor this so-called Megamaser emission show enhanced core activity in the form of a nuclear starburst or an active-galactic-nucleus. The exceptional maser properties together with the nuclear activity indicate that the line radiation originates in the circumnuclear environment close to the central engine. The environment for producing maser emission in our Galaxy fulfills some unique requirements that will be compared with those of the extra-galactic Megamaser emission. Using very-long-baseline-interferometry, the observational data show that the radio and the molecular line emission structure reveal a rather more complex picture of the circumnuclear environment where the masers occur. At such scale-sizes the individual Megamaser galaxies display diverse maser- and nuclear properties, which all contribute to the understanding of the molecular environment in active nuclei.