| الوصف: |
Observations of galaxies showing signatures of recently finished nuclear activity are reviewed, and it is shown that the available evidence supports the hypothesis that nuclear activity in massive galaxies is a recurrent phenomenon. This fact can be rather simply understood on a model in which stellar mass loss from the inner bulge components of galaxies (r (-I kpc, say) flows inwards to form a dense large-scale nuclear disc, whose evolution then leads to periodic phases of nuclear activity. The structure and evolution of the disc are considered, and it is shown that the temperature is determined mainly by the heating effect of infalling matter. Even if this gas should be hot (T - 2 x 101 K, say), the disc density is sufficiently high that its equilibrium temperature remains quite low. Close to the centre most of the gas is probably in molecular form. The disc grows slowly in mass until the onset of a star-forming gravitational instability. At this point, strong turbulence driven by massive early-type stars leads to a rapid inward (viscous) transport of matter, which in turn leads to the formation of an active nucleus. In the first instance the disc forms a low-mass (M S 106 Me) low-entropy spinar, and the evolution of this object might in principle explain even the most luminous quasars. The late evolution of the system depends crucially on the late evolution of the spinar, and the two possibilities (1) that the spinar undergoes gravitational collapse, and (2) that the spinar undergoes a disruptive nuclear explosion can not yet be distinguished theoretically. The theory is based on parameters appropriate to our own Galaxy, and observations of our Galactic Centre indicate that it in in a post-active state. The apparent absence of a supermassive black hole (M ~ 5 x 106 Me) argues against the black hole hypothesis, and it is tentatively concluded that the spinar's late evolution results either in a disruptive nuclear explosion, or in disruption from a state more tightly bound than Enuc ("' 0.7 % Me 2) by processes involving still undiscovered physical laws. The detailed evolution of low-mass spinars in normal galactic nuclei is thus an important theoretical problem for future investigation. |
