Towards a Truly Unified Model of AGN
Accepted for publication in PASA
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.
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