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Since the energy output of an AGN is too powerful to be related to stellar activity, and being ~ 10 times more efficient than nuclear fusion at converting rest mass into radiation (e.g. Frank, King & Raine 1992) accretion onto a super-massive (gtapprox 106 Msun) black hole is generally proposed as the source producing the observed power within a sufficiently small volume (Salpeter 1964; Zel'dovich & Novikov 1964; Begelman, Blandford & Rees 1984; Rees 1984). Until recently, however, firm observational evidence on the existence of black holes had been lacking, but in the past few years detailed studies of astronomical mega-masers have given direct evidence of the existence of these objects.