| © CAMBRIDGE UNIVERSITY PRESS 1991
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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.
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
| (1) |
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.
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).
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).
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
| (2) |
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.
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).