The black hole masses are estimated in chapter I of this
thesis following a different approach, i.e. through the study of
the statistical properties of the collectivity of AGNs: space
density and evolutionary (the redshift dependence) behaviour. We
also derive from the observations constraints on the radiation
mechanisms and on the physical conditions in the emission region,
which then may favour one or another of the proposed central
engines.
In chapter II we present the basic theory of non- thermal
radiation processes and relativistic beaming. The emphasis is now
placed on the importance of the viewing angle for the observed
global energetics of individual objects and for the statistics of
AGNs.
The physics of accretion flows onto massive black holes is
described in chapter III. The control parameter is now the
accretion rate. The general approach is to consider as fundamental
the problem of energy generation and try to derive the main
features of the emission spectra associated with different
accretion regimes.
The thermal spectrum of a radiation-supported torus is
computed in chapter IV. Again, the main feature is found to be the
dependence of the observed spectrum on the viewing angle. The
occultation of the innermost disk region due to self-shadowing and the enhancement of the radiation field which results from the
multiple scatterings of photons off the funnel walls. are
self-consistently taken into account.
A modest 'unified scheme' is suggested on the basis of this
spectral behaviour. Basically, radiation tori would represent the
powerhouse of optically bright QSOs. High and low- inclination
systems would be responsible, respectively, for the optical/UV and
UV/soft X-ray thermal excesses which are observed in their
spectra. The shielding of the outer regions from the primary
ionizing radiation source may explain the strong, low-excitation
Fe II emission lines which are typically observed in these
objects.
