Towards a Truly Unified Model of AGN:
Aspect, Accretion and Evolution

Michael A. Dopita, PASA, 14 (3), 230
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Conclusions

In this paper, I have established the feasibility of the hypothesis that galactic mergers determine, as much as does orientation, the nature and evolutionary stage of the AGN that we observe. The growth of the BH during a major merger event determines the transformation of the nuclear BH from those characteristic of Seyfert galaxies to those such as are found in ellipticals. In the case of a merger event between two gas-rich systems, the BH of the merging galaxies can grow from tex2html_wrap_inline1051 10tex2html_wrap_inline1337 to tex2html_wrap_inline1051 10tex2html_wrap_inline933 . It is also known that such mergers can produce the tex2html_wrap_inline899 luminosity profile characteristic of Elliptical galaxies (Barnes and Hernquist, 1996). Such mergers also produce a massive star burst, linking them with the luminous and ultra-luminous infrared galaxies. The size of the BH that can be grown has also been shown to depend on the total gas supply, and therefore upon the mass of the combined system.

For much of the early growth phases, the supply of gas to the region of influence of the BH is likely to be super-Eddington. We show that this is likely to produce an optically-thick radiatively-driven wind. This wind may generate the `big blue bump' in the continuum spectrum, photoionize the broad line region, and may be sufficient to choke off the relativistic jets in this phase, producing a radio-quiet AGN. The fast wind will also interact with the circumnuclear material to form strong shocks which are probably the source of Fermi acceleration of electrons needed to produce the radio core.

For sub-Eddington accretion rates, such as found in either elliptical mergers or in cooling flows, the radio jets can freely escape from the thin accretion disk, so radio-loud AGN and relativistically-boosted BL Lac /Blazer phenomena will be preferentially produced in this case. The infalling gas is shocked as it settles down towards the thin inner accretion disk, and it is these shocks which probably produce the classical LINER emission.

This paper has established the outline of of a grand unified model, but fails to provide enough detail of the various physical processes invoked. This will have to be the subject of later papers. The following theoretical and observational studies would do much to establish the plausibility (or otherwise) of the model:

  • What are the details of the gas dynamics in the inner accretion core? What is the density, mass and size of the molecular clouds here?
  • How does the BH interact with these clouds to move into and graze upon them? What is implied about the duty cycle of AGN activity in this case? Do we get repeated episodes of radio jet activity as the AGN moves from on dense cloud core to another?
  • What is the competition between the gas infall, the growth of the BH, the disruption of the molecular clouds produced by BH winds and jets, and the fragmentation of the molecular clouds into stars? Can this explain the relationship between the BH mass and the core mass which appears to be observationally established through the work of Lonsdale, Smith and Lonsdale (1995) and Nelson and Whittle (1996)?
  • What is the form of the ``big blue bump'' generated by an optically-thick electron scattering photosphere, and can this reproduce the observations?
  • What emission line spectrum would this produce by photoionisation of the surface layers of the inner accretion disk, and does this reproduce the observations, including the constraints imposed by the reverberation mapping analyses of nearby Seyfert I galaxies?
  • What is the synchrotron spectrum produced in the shock generated at the thermal wind / disk interface, and can this explain the radio core observations?
  • What is the evidence for hot, thermal plasma in the NLR lobes of radio-quiet objects?
  • What are the details of the jet / thermal wind interaction? Can we bury this interaction below a free-free photosphere?
  • To what extent is the radio-loud / radio-quiet dichotomy produced by selection effects in the samples used to generate figure (1)? We need an unbiased deep search for radio emission associated with a large optically identified sample of AGN, or a similar study involving faint IR sources that fall below the IRAS sensitivity limit in order to establish the true relative space density of radio-intermediate objects.
  • Do radio-loud and radio-quiet QSO host galaxies differ in any morphological sense? In our model, the radio-loud objects should look more like normal ellipticals, being in a late merger or post-merger phase, while radio-quiet object should appear more frequently in merger systems.
  • According to our jet-driven model, the jets of high-redshift radio galaxies should be confined by the higher ISM density, and in consequence should be shorter, but of intrinsically higher surface brightness (and, possibly, luminosity) than those in more nearby systems. Is this observed?

Next Section: Acknowledgements
Title/Abstract Page: Towards a Truly Unified
Previous Section: Evolution and Aspect
Contents Page: Volume 14, Number 3

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