Annu. Rev. Astron. Astrophys. 1982. 20: 431-468 Copyright © 1982 by Annual Reviews. All rights reserved

5. COSMOLOGICAL EVOLUTION OF ACTIVE NUCLEI

Galaxies and their environments have evolved significantly since z ~ 3 to 4. The question of this evolution has recently been discussed by Longair (1978), Schmidt (1978), Schmidt & Green (1980), Schmidt (1981), and Wall (1981) on the basis of quasars and radio sources. Turner (1980b), Avni (1981), and Tyson (1981) have raised the caveat of gravitational lensing. To summarize: High-luminosity active nuclei were much more common in the early Universe. Less luminous sources evolve more slowly with look-back time or not at all. The evolution, however, must cease at times before z ~ 3.5. Optically selected QSOs evolve more rapidly with z than do radio-selected quasars. Steep-radio-spectrum QSOs may evolve more rapidly than flat-spectrum sources, although this point is disputed (Wall 1981).

Models of the origin and evolution of activity in galaxies must be able to account for the evolution of the strength and number density of galactic activity in the past. Clearly galaxy-galaxy interaction models, ICM accretion models, and other models sensitive to the density of the extragalactic environment will predict evolution of activity at earlier cosmological eras. Stocke & Perrenod (1981) have been able to set some constraints on such processes related to the evolution of QSOs. Cosmological implications of internally generated galactic activity in an evolving Universe have been addressed by McMillan et al. (1981), who show that quasar evolution can be reproduced by a model involving the dynamical evolution of a nuclear star cluster and a massive black hole. Roos (1981a) has investigated the cosmological evolution of the galaxy merger rate and its implications for nuclear activity.