|| © CAMBRIDGE UNIVERSITY PRESS 1991
The literature on models for various aspects of AGN is immense, even after removing a number of extremely implausible suggestions. Thus it is not possible to do justice to all, or even most, of the serious ideas that have been put forward. Recent reviews that cover some of the same material discussed in this section are Begelman (1985); Begelman, Blandford & Rees (1984); Blandford (1986); Phinney (1986); Rees (1984a, b; 1986) and Wiita (1985). Kundt (1987) stresses and Wiita (1985) covers some of the more unorthodox proposals such as magnetoids, supermassive discs and white holes; while some of those ideas deserve more attention, they will not get it here.
2.1. Why supermassive black holes?
Many lines of evidence support the argument that SMBHs are at the cores of all AGN. Recent reviews of this evidence include Wiita (1985); Blandford (1986); and Trimble & Woltjer (1986), and here we will just briefly mention some of these points.
2.1.1. Rapid variability
Significant changes are often seen over days in the optical and over hours in the X-ray parts of the spectrum, strongly suggesting extremely compact active regions. Smaller, but still significant, optical variability over periods of tens of minutes in BL Lacertae objects has recently been firmly detected by Miller et al. (1989). The radius of the event horizon of a black hole of mass MBH and angular momentum parameter a is
and in the absence of relativistic effects the minimum time for variability would be t 2cRs. Relativistic beaming effects could allow for the variations we observe at a distance apparently to be faster than those seen by an observer in close proximity to the SMBH (cf. Chapter 4). However, the inclusion of these (always uncertain) effects would nominally allow even higher mass objects to be at the centre.
2.1.2. Emission line widths
The broad emission line regions of quasars and Seyferts frequently correspond to spreads in velocities of greater than 5000 km s-1. The shapes of these lines and the fact that most of the continuum emission escapes strongly imply that they are due to a large number of small clouds. The relative strengths of different lines enable densities of between 1010 and 1013 cm-3 to be determined and X-ray absorption measurements indicate column depths of about 1022 to 1023 cm-2, implying radii of about 1011 to 1013 cm and temperatures of ~ 104 K (e.g., Matthews & Capriotti 1985; Krolik 1988). Both variability measurements and photoionization models relying on the observed AGN continuum imply that this broad line region (BLR) is located within about (L46)1/2 pc from the very centre of the AGN, where L46 = Lionization continuum/1046 erg s-1. While an entire industry has grown up around the manufacture of detailed models of the formation and motion of these BLR clouds, for our current purposes it does not matter whether they are infalling, outflowing or orbiting: the large velocities at those rather small distances indicate the presence of contained masses consistent with very massive SMBHs. Recent spectroscopic observations of broad line variability in the Seyfert galaxies Akn 120 (Peterson et al. 1985) and NGC 5548 (Peterson & Ferland 1986; Peterson 1987) indicate that the BLR is more compact (a few light weeks) than normally assumed; in this case the line widths imply SMBHs of ~ 108 M. Some of the possible interactions of jets with the BLR will be discussed in Section 3.3.
Narrow emission lines (velocity spreads of 300-1500 km s-1) are normally assumed to originate well outside the compact core, and it has been convincingly demonstrated that a large fraction of galaxies contain unusual low intensity emission line regions within their inner kpc or so. Such "LINERs" (e.g., Keel 1985) have now been shown frequently to exhibit broad wings around their narrow lines; these wings have been used to support the claim that SMBHs are extremely common in galactic nuclei (Filippenko & Sargent 1985). Further, the marginally detected relative redshift of broad Balmer emission lines with respect to forbidden narrow lines can be attributed to the general relativistic gravitational redshift expected from the proximity of the BLR clouds to a SMBH (e.g., Peterson 1987, 1988).
2.1.3. Radio observations
Several types of radio observations strongly support the SMBH idea. First, the frequency of good alignments between small scale and large scale jets (cf. Section 4.4) implies that their source has a good memory of direction for at least 106 yr. This long term stability is most naturally attributed to the spin axis of a SMBH. Second, regardless of which model for relativistic beaming is adopted, the probability of very rapid bulk motion fits naturally into a picture where a deep relativistic gravitational well is available. It is very difficult to conceive of a mechanism that can accelerate significant quantities of matter to velocities comparable to c without involving relativistic potential wells, which, in the case of an AGN, almost certainly has to be a SMBH. Finally, VLBI observations of NGC 1275 have shown a very small elongated central component perpendicular to the small-scale jet; this may well correspond to optically thick synchrotron emission from a disc extending ~ 103Rs from an ~ 109 M BH (Readhead et al. 1983).
2.1.4. Power and efficiency
The tremendous power emitted by quasars, up to 1048 erg s-1 provides yet another indirect argument in favour of this picture. The standard argument notes that outward radiation pressure drives out any infalling matter when the luminosity exceeds the Eddington limit
the opacity is expected to be due to electron scattering at the conditions encountered in an AGN, so that is taken as a constant. This corresponds to the conversion of E LE/c2 completely into radiant energy.
Since nuclear fusion allows less than 1% of mass to be converted into energy, relying on that source demands incredibly high mass fluxes (hundreds of solar masses per year). However, accretion onto BHs can in principle allow the radiation of up to 42% (although more probably no more than 32%; see Section 2.2.1) of the rest mass energy of infalling matter, substantially reducing the necessary influx. Since some outbursts apparently correspond to the entire rest energy of a star, even extremely enhanced supernova rates seem incapable of providing a viable explanation, while the tidal disruption and swallowing of a massive star captured by a SMBH or a super-flare associated with a disc around one might possibly fit the bill. Still, it should never be forgotten that the huge energy requirements depend upon the now well-confirmed claim that quasars are at the large distances demanded by the cosmological interpretation of their redshifts and upon the less certain assumption that the overall emission is roughly isotropic.
2.1.5. Stellar dynamics
Steep rises of the velocity dispersion of stars towards the centres of galaxies and steep gradients in stellar velocities across the galactic nuclei attributed to rotation provide dynamical evidence in favour of massive, non-stellar, objects at the centres of a growing number of galaxies. Continuation of the rising surface brightness of galaxies towards their very centres also supports this hypothesis. The difficulties in obtaining high spatial resolution and removing the blurring effects of seeing on the spectra are tremendous, and mean that early conclusions that the active galaxy M87 contains a 3 × 109 M BH (Young et al. 1978; Sargent et al. 1978) are weak in that anisotropic velocity distributions can also explain the observations (e.g., Binney & Mamon 1982). However, the case based on velocity dispersions and rotation curves for SMBHs of around 106 - 7 M in nearby non-active galaxies such as M31 and M32 is quite impressive (Dressler 1984; Tonry 1984; Dressler & Richstone 1988). The evidence in favour of a similarly massive object at the Galactic Centre will be mentioned in Section 10.5.
2.1.6. Evolution of galactic cores
Although it would be impossible to convincingly compute every possible fate for a dense star cluster at the centre of a nucleus, the general consensus (e.g., Begelman & Rees 1978; Wiita 1985) is that such a cluster is unstable and much of it will eventually collapse into a SMBH. Various massive objects supported by rotation and/or magnetic fields could certainly exist, and might well play roles in energizing AGN; however, they are probably transient phenomena and unlikely to be the dominant prime mover. Recent numerical simulations of the evolution of very dense clusters of solar mass sized compact objects explicitly illustrate the formation of a trapped surface, i.e., black hole (Shapiro & Teukolsky 1985a, b; Kochanek et al. 1987), adding strength to this theoretical argument.
Some very plausible evolutionary scenarios for AGN in terms of the various types of accretion to be discussed in Section 2.2 and 2.3 have been proposed (e.g., Blandford 1986). Such pictures involve the growth of BHs in the vast majority of galaxies, with quasars and bright Seyferts corresponding to those SMBHs that are currently being fed at high rates (comparable to the Eddington limit), and weak Seyferts, LINERS and radio galaxies being associated with subcritically fed holes. Although greatly simplified, such a scenario seems to fit observations of the cosmological evolution of the comoving densities of Seyferts, quasars and radio galaxies surprisingly well (Blandford 1986).