Theoretical models of active galactic nuclei

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.