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
(
106
M
) 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.