Active galactic nuclei (AGNs) are galaxies whose nuclei are highly
luminous, with spectra showing broad emission lines covering a wide
range of ionization. AGN activity spans a broad range of luminosity,
from the brightest quasi-stellar objects (QSOs) to the more common but
less luminous Seyfert galaxies. Two significant facts about AGNs make
them very interesting to study. The first is their ability to generate
extraordinary luminosities (up to 1047 ergs s-1)
in tiny volumes
( 2 ×
1014 cm; see, e.g.,
Edelson et al. 1996).
The second is the similarity in their spectral features over 7 orders of
magnitude in luminosity, across most of the electromagnetic spectrum. No
other phenomena have these unusual properties. The similarity in
spectral properties over many orders of luminosity indicates that the
central engine in AGNs must have the ability to scale with
luminosity. It is widely believed that accretion of gas into a central
supermassive black hole lies at the heart of the phenomenon (see, e.g.,
Rees 1984).
The accretion flow is thought to be the source of the X-ray, ultraviolet
(UV), and optical continuum emission, which ionizes circumnuclear gas in
both the broad-line region and narrow-line legion, and may also be the
launch site for winds and jets. Well outside the broad-line region (BLR)
lies a dusty torus (or perhaps a warped disk) that obscures some lines
of sight to the nucleus and accounts for some of the observed diversity
in the AGN phenomenon
(Antonucci 1993).
Readers interested in a broad overview and introduction to the field of
AGNs can find several good textbooks, including
Peterson (1997) and
Krolik (1999).
The standard picture of the central engine of AGNs now has limited
observational support. On the smallest scales from Hubble Space
Telescope (HST) imaging and spectroscopic observations, there
is evidence for the existence of massive compact objects at the centers
of galaxies
(Kormendy et al. 1996a,
1996b, and
1997;
van der Marel et
al. 1997;
Ford et al. 1994;
Harms et al. 1994;
Macchetto et al. 1997;
Bower et al. 1997;
van der Marel & van
den Bosch 1998;
Richstone 1998;
Magorrian et al. 1998).
Very broad Fe K lines
observed by the Advanced Satellite for Cosmology and Astrophysics
(ASCA) indicate the presence of gas moving in a relativistic
potential well
(Tanaka et al. 1995;
Mushotzky et al. 1995;
Nandra et al. 1997b).
On parsec scales, HST imaging observations
(Walter et al. 1996;
Ford et al. 1998)
and water megamaser observations of a handful of nearby AGNs show a disk
structure, possibly the thick molecular torus
(Nakai, Inoue, &
Miyoshi 1993;
Greenhill et al. 1995;
Braatz, Wilson, &
Henkel 1997).
Despite the fact that much of the large-scale structure has now been observationally verified, we still do not understand the central engine itself. The main difficulty in achieving this goal is that the central accretion flow, which is responsible for the observed optical, UV, and X-ray spectrum, is too small (~ 10 RG = 5 × 10-5 pc for a 108 solar mass black hole, where RG = GM / c2 is the gravitational radius of the black hole) to resolve at the distances of even the nearest AGN. (There is one interesting exception: the Einstein Cross, which we discuss in Section 3.6 below.) Therefore, we are forced to rely on theoretical models, their predictions, and comparison with observations in order to make progress.
The theoretical accretion models are distinguished by the assumed flow
geometry, the state of the plasma within the flow, and the mechanism
whereby gravitational energy is converted into observed radiative and
kinetic power. The simplest flow geometry is spherical, but such flows
are generally very inefficient radiators unless there is some internal
source of dissipation, e.g., shocks
(Mészáros
& Ostriker 1983).
Observations on larger scales than the central accretion flow suggest
that spherical symmetry is probably broken and that a well-defined axis
is present in the central engine. These include the evidence for
large-scale gaseous disks as discussed above, as well the presence of
jets in both radio-loud and radio-quiet AGNs
(Livio 1997;
Blundell & Beasley
1998).
The axis is generally believed to be associated with the angular
momentum of the accretion flow and the hole itself. The specific angular
momentum of gas in the central regions of galaxies generally greatly
exceeds that of a particle in a circular orbit near the black hole (~
RG c). To have less than this critical
value of angular momentum, material 1 pc away from a 108
M black
hole would require nonradial velocity components less than
1 km
s-1. Centrifugal forces on material of fixed specific angular
momentum vary with radius r as r-3, while
gravity varies as r-2. Therefore, unless angular
momentum is transported outward on a dynamical timescale as material
flows inward from large scales down to the black hole, the flow must
become rotationally supported and form an accretion disk. As
first pointed out by
Salpeter (1964) and
Zeldovich & Novikov
(1964),
accretion disks can provide a high efficiency of matter-to-energy
conversion in a very small volume. The outward transport of angular
momentum occurs slowly through poorly understood dissipative processes
("viscosity"), thereby converting a significant fraction of
gravitational energy into heat, magnetic fields, and/or outflowing
kinetic energy. It is this largely theoretical reasoning that has
made disks so ubiquitous as models of the central parts of the accretion
flow in AGNs.
The most common assumption about the state of the plasma within the disk
is that it is optically thick and thermal. This immediately implies that
a substantial fraction of the bolometric power should be in the form of
UV photons: a blackbody emitting at a significant fraction of the
Eddington luminosity on size scales associated with supermassive black
holes has a temperature in the UV range. In fact, a lot of power in AGNs
is emitted in the optical/UV region of the spectrum (the Big Blue Bump),
but the full spectral energy distribution (SED) is rather more
complicated than that. As shown in
Figure 1, the overall broadband AGN continuum is
relatively flat in
F
and
extends over nearly 7 orders of magnitude in frequency, which implies
that approximately the same amount of energy is emitted per decade of
frequency. For radio-quiet AGNs, this broadband spectrum can be divided
into three major components: the infrared bump, the Big Blue Bump, and
the X-ray region.
The Big Blue Bump (BBB) continuum component in AGNs extends from the near-infrared at 1 µm to past 1000 Å in the UV and in some cases apparently all the way to the soft X-ray region of the spectrum. More than half the bolometric luminosity of an unobscured AGN is typically emitted within this spectral range, and thus its origin is directly related to the main energy production mechanism. The BBB is thought to arise from an accretion disk and is the spectral region that we will mostly be discussing in this paper. We discuss the observed properties of the BBB extensively in Section 3.
The broad infrared bump extends from ~ 100 to ~ 1 µm. Quasars show a deep minimum at 1 µm and have a sharp cutoff in the submillimeter. In Seyfert galaxies, the 1 µm minimum is rarely detected because of galaxy starlight contamination (Neugebauer et al. 1987; Sanders et al. 1989). Prior to longer wavelength observations, it was thought that the near-infrared emission in radio-quiet AGNs could be due to synchrotron emission, which could extend as a power law to higher photon energies underneath the BBB. Today, however, the infrared bump is generally thought to arise from reprocessing of the BBB emission by dust with temperature in the range of 10-1800 K and at a range of distances from the central UV source (Barvainis 1987, 1990; Sanders, Scoville, & Soifer 1988; Phinney 1989). The spectral trough at 1 µm is then naturally interpreted as being due to the finite sublimation temperature (~ 1800 K) of dust. Additional evidence for the dust interpretation is the fact that the infrared hump shows no rapid variability (see, e.g., Hunt et al. 1994) and that the long-wavelength turnover in the submillimeter is very steep (see, e.g., Hughes et al. 1993). Observations of a handful of radio-quiet AGNs in the far-infrared using the Infrared Space Observatory show strong evidence for thermal emission (Haas et al. 1998). In the radio-loud AGNs, the infrared emission is a mixture of thermal and nonthermal radiation, but in most cases one of the two components is dominant (Haas et al. 1998).
In the soft X-ray region, many objects show a "soft X-ray excess," which
is emission that exceeds the extrapolation from the observed hard X-ray
power-law continuum
(Turner & Pounds
1989;
Masnou et al. 1992).
This excess is subject to a number of observational difficulties and is
probably the least well understood component of the AGN's spectral
energy distribution (SED). It may be that it is the high-energy
continuation of the BBB, but this interpretation is not
straightforward. We discuss the details in
Section 3.1 below. The characteristics
of the soft X-ray excess in both Seyfert galaxies and quasars are not
well quantified. Radio-quiet QSOs have a mean energy spectral index
(F
X) of
X = -1.72
± 0.09 in the soft X-ray band
(Laor et al. 1997),
while Seyfert 1 galaxies have
X ~ -1.37
(Turner, George, &
Mushotzky 1993;
Walter & Fink
1993).
In the hard X-ray band, the AGN spectrum consists of a power law with
energy spectral index ~ -0.9. This power law is almost universally
believed to be due to Compton upscattering of optical/UV photons by hot
or nonthermal electrons somewhere in the central engine, possibly a hot,
magnetized corona above the disk. Superposed on this power law are an
Fe K emission line at
6.4 keV and a Compton reflection hump above 10 keV, at least in the case
of some Seyfert galaxies
(Pounds et al. 1989;
Nandra & Pounds
1994).
These two features are thought to be due to fluorescence and reflection
from "cold" material, possibly the accretion disk itself in the case of
Seyfert 1 galaxies. This is supported by the fact that the
Fe K
emission line is
observed to be very broad
(Tanaka et al. 1995).
Both the reflection hump and the hard X-ray power law extend up to high
energies before cutting off at around several hundred keV
(Gondek et al. 1996).
The hard X-ray properties of high-luminosity AGNs (QSOs) are not as well
understood, partly because they are relatively faint in the hard X-rays
given their optical/UV luminosity compared to Seyfert galaxies. The
Fe K
line and reflection
hump are relatively weak or nonexistent
(Iwasawa & Taniguchi
1993;
Nandra et al. 1995;
Nandra et al. 1997c).
It is the purpose of this paper to review the current status of the observational and theoretical research effort to understand the central accretion flow and its relation to the BBB. We focus almost exclusively on radio-quiet AGNs, whose emission is dominated by the accretion flow. Radio-loud objects can also have substantial contributions to their observed emission from jets and outflows, and we refer the reader to the excellent review of Urry & Padovani (1995) for a discussion of their properties. While we will mention the important clues being provided by other regions of the electromagnetic spectrum, particularly X-rays, we concentrate here on the optical/UV BBB. We begin Section 2 of the paper by discussing the early, simple accretion disk models that were first applied to the AGN problem. These models have proved to be very inadequate, and we discuss the main observational problems in Section 3. We then turn to more recent efforts to improve these models and how they fare against the observations in Section 4. We discuss alternatives to the standard accretion disk paradigm in Section 5. Finally, we summarize our main conclusions in Section 6, pointing out the main research directions, both theoretical and observational, which we feel will lead to further progress in our understanding of the primary power source of AGNs.