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

Michael A. Dopita, PASA, 14 (3), 230
The html and gzipped postscript versions of this paper are in preprint form.
To access the final published version, download the pdf file
.

Next Section: Conclusions
Title/Abstract Page: Towards a Truly Unified
Previous Section: Growth of Black Hole
Contents Page: Volume 14, Number 3

Evolution and Aspect

The evolutionary sequence has already been suggested on the basis of Figure 1. In the case of a major merger event between two massive gas-rich systems, the initial phase would be a galaxy-wide starburst producing a luminous infrared galaxy with HII-like characteristics. In the later phases, gas flow toward the nucleus feeds the BH and outflow could then excite the Seyfert 2 - like emission seen in some of the very distant ultraluminous IRAS galaxies (Rowan-Robinson, et al. 1991, 1993; van Ojik 1994). Such mergers will initially be radio-quiet, since the supply of gas into the nuclear regions is sufficient to choke off high-luminosity radio jets, but as nuclear accretion rates fall, radio jets might later escape to produce a high z radio source with its strong narrow-line emission region (Meisenheimer, Hippelein & Neeser 1994; McCarthy, Spinrad and van Breugel 1995). As mentioned above, such mergers of gas-rich systems would have been much more common in the early universe and especially at the epoch of formation of the Abell clusters. Since this epoch was so early, z tex2html_wrap_inline929 2-4, the merging gas-rich systems may not even have had time to form regular spiral galaxies. However, recognising this, let us call them `spirals' for convenience.

It is therefore reasonable to propose the following evolutionary scenario:


eqnarray528

Such `primary' merger events will give rise to the QSOs and the majority of elliptical galaxies. In the event of a later merger event:


eqnarray536

Whether evolution to the last of these steps can occur is not clear, but the sharp break between the FR 1 / FR 2 classes (Owen & Ledlow 1994), which can be explained theoretically (Bicknell, 1994), is certainly suggestive. Alternatively, the case of M87 suggests that low-power radio sources may be fed by a subsequent accretion episode at sub-Eddington rates, such as feeding through the cooling flow like M87. In this case the evolutionary scenario would be:


eqnarray551

In this scenario, evolution determines the nature of the AGN, but aspect determines the type of AGN we see from outside. In Figure 4, I suggest a taxanomic ordering of the various classes of AGN based on their orientation, the nature of their feeding, the mass of the central BH, and the accretion parameter, given in Figure 2, which measures the rate of feeding of gas into the nuclear regions compared with the Eddington value. We have already distinguished three classes of feeding, major merger-driven inflow in gas rich systems, merging of an elliptical with a low mass companion, and feeding through cooling flows. To these we should add feeding by flows driven by the development of bar-like instabilities in spiral galaxies, which may be major cause of Seyfert activity. Of course, such bar-like instabilities may also be produced by the tidal forces generated in a close encounter with another galaxy. In figure 4, I suggest evolutionary paths, and identify prototypes for various classes of AGN. Note that in Seyferts, the opening angle of the outflow (``ionisation'') cone increases with time as the accreting gas is cleared away, so that the Sy I class may be observed over a wider range of angles is more evolved sources. In general, radio-loud objects, and AGN showing relativistic beaming effects will only be seen for sources lying in the bottom right-hand corner of these diagrams.


Next Section: Conclusions
Title/Abstract Page: Towards a Truly Unified
Previous Section: Growth of Black Hole
Contents Page: Volume 14, Number 3

Welcome... About Electronic PASA... Instructions to Authors
ASA Home Page... CSIRO Publishing PASA
Browse Articles HOME Search Articles
© Copyright Astronomical Society of Australia 1997
ASKAP
Public