Published online by Cambridge University Press: 14 August 2015
Extragalactic masers were discovered more than 20 years ago (Whiteoak & Gardner 1973). It soon became apparent that those extragalactic masers we can detect are intrinsically very powerful and are located in galactic nuclei. The term Megamaser was coined to describe the most luminous OH masers, which are a million times more powerful than any OH masers within our own Galaxy (Baan & Haschick 1984). The same word is nowadays applied to any powerful maser associated with an active galactic nucleus (AGN). Whereas normal Galactic masers would be barely detectable outside the Local Group, megamasers are detectable in principle out to redshifts of z ~ 2 (Baan 1997). The galaxies hosting megamasers in their nuclei are invariably active in some degree: they include ultraluminous infrared galaxies, starbursts and Seyfert galaxies.