Evolutionary scheme for active-nucleus objects

The features typical of active nuclei in galaxies and the conditions necessary for activity are discussed. QSO's may represent nuclei of luminous galaxies in an early, particularly violent evolutionary phase. The spectroscopic properties of the various species of quasar-like objects suggest a possible evolutionary sequence from those with Sy 0, I (Seyfert 1) spectra to Sy II, III spectra. An evolutionary scheme is outlined whereby the active nuclei in various types of galaxies would correspond to well-evolved QSO's belonging to either spheroidal [QSO(E)] or flat [QSO(S)] systems. These two kinds of QSO's may be either strong radio emitters [QSS(E), QSS(S)] or weak radio emitters [QSG(E), QSG(S)], a distinction which would manifest itself, in evolutionary terms, as the difference observed between the radio properties of radio galaxies and Seyferts, on the one hand, and normal E, S galaxies with low-activity nuclei, on the other. QSO(S)'s should be scores of times as common in the universe as QSO(E)'s; there may be as many as 10-6 QSG(S)'per Mpc3. Such objects should contribute significantly to counts of blue 21/sup m/--23/sup m/ starlike images