Towards a truly unified model of AGN: Aspect, accretion and evolution

Abstract

The Unified Model holds that the aspect-dependent effects primarily determine the nature of the active galactic nucleus that we observe. In this paper, I argue that three parameters; aspect, accretion rate into the nuclear regions, and the evolutionary status of the central black hole hold the key to unification. The mystery of why the epoch of quasar formation occurred so early in the evolution of the Universe, why radio-loud QSOs represent only a small fraction of the general population of QSOs, and why ellipticals are invariably the hosts of radio-loud active galaxies could be explained if (a) the most rapid growth of black holes occurred in galactic merger events, and if (b) an excess in the rate of nuclear feeding was able to choke off the radio jets, producing radio quiet QSOs. In this paper, I develop the idea that rate of nuclear feeding plays a dominant role and that feeding at super-Eddington rates into the broad-line region (BLR) during merger events is the means whereby massive black holes are grown. In particular, I develop a toy model for the radio-loud, radio-quiet dichotomy based on the rate of nuclear feeding, suggest an electron scattering model for the 'big blue bump' and its relation to the BLR, and emphasise the important diagnostic capabilities offered by analyses of the narrow line regions based on shock excitation models.