The structure and physics of AGNs

5.5.1 An overview

The current paradigm for explaining the structure of AGNs is based on a combination of relatively straightforward physical considerations and some 40 years of sometimes ambiguous and hard-to-interpret observational data. While many important details are not understood, even some fundamental problems remain unsolved (e.g. how does mass

Figure 5.4. The classic unification model for (in this case, radio-loud) AGNs. Surrounding the black hole is a luminous accretion disk, with radio jets emitted along the axis of the system. On large scales, there is a thick dusty torus (the prominent feature in this diagram) that obscures the inner part of the torus along lines of sight near the midplane, thus blocking the direct view of the accretion disk and BLR, both of which are within the inner part of the torus. Narrowline emission arises on larger scales and can be seen in all directions. Free electrons outside the torus can scatter light back towards an observer along the midplane; the inner regions are then visible in the scattered, polarized light. From Urry & Padovani (1995). Copyright PASP, with permission of the authors.

Figure 5.4. The classic unification model for (in this case, radio-loud) AGNs. Surrounding the black hole is a luminous accretion disk, with radio jets emitted along the axis of the system. On large scales, there is a thick dusty torus (the prominent feature in this diagram) that obscures the inner part of the torus along lines of sight near the midplane, thus blocking the direct view of the accretion disk and BLR, both of which are within the inner part of the torus. Narrowline emission arises on larger scales and can be seen in all directions. Free electrons outside the torus can scatter light back towards an observer along the midplane; the inner regions are then visible in the scattered, polarized light. From Urry & Padovani (1995). Copyright PASP, with permission of the authors.

accrete onto the black hole, and how are jets formed?), our view of the basic structure of active nuclei, which we outline below, has been fairly secure and stable for at least the last two decades.

The major components of a classical active nucleus (Figure 5.4) are, from smallest to largest physical scale, a central supermassive black hole with a surrounding accretion disk that produces the X-ray through optical continuum, a broad-line region (BLR), and an extended narrow-line region (NLR) whose inner part is on the same spatial scale as an opaque disk-like structure, sometimes referred to as an "obscuring" (or "dusty") torus, which extends out to kiloparsec scales. On even larger scales, radio jets are sometimes seen along the axis of the system.

5.5.2 The supermassive black hole 5.5.2.1 Physical considerations

Supermassive black holes are thought to reside at the centers of all galaxies, at least of those with a nuclear bulge. About 5%-10% of bright galaxies harbor luminous active nuclei,t those black holes that are actively accreting surrounding matter, which is heated as it falls into the deep gravitational potential of the black hole.

It is easily demonstrated that the central black holes must be massive. Consider a quasar of black-hole mass MBH and luminosity L that emits isotropically. At some distance r from the center, the energy flux is F = L/(4^r2). Since a photon's momentum is t This is a rather luminosity-dependent statement. While only about 5% of bright galaxies are Seyfert galaxies, as many as ~ 40% are LINERs.

related to its energy by p = E/c, the momentum flux, or radiation pressure, due to the quasar at r is thus

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