Peter Barthel
Discovering and investigating the various properties of QSOs and
active galaxies such as radio galaxies, Seyfert galaxies, and BL
Lacertae objects objects over the past three decades, astronomers
noted that different classes of objects share properties, even in a
quantitative sense. This led them to propose interrelating schemes for
seemingly different objects. Some of these schemes had to be
abandoned; other ones survived, sometimes after minor or major
revision. This entry deals with such interrelating and/or unifying
schemes. Because knowledge of the similarities and dissimilarities of
the various types of QSOs and active galaxies is essential to
understanding possible interrelations, I will first briefly introduce
the subject and review the basic, relevant properties of the different
classes of objects.
Normal galaxies such as our Milky Way galaxy or the Andromeda galaxy
(M31) emit the combined radiation of some hundred
billion stars as the
bulk of their radiation. The evolution of such normal galaxies is a
gradual one, governed by the evolution of these very many stars, which
are these very many stars, which are born, go through various phases
of stella stellar life, and finally die. Unless major disturbances
from outside take place, the structure and radiation of a normal
galaxy will not change during a time span of a hundred million
years. Active galaxies produce more than just the radiation of
stars. Radio and x-ray radiation from nonthermal processes, but also
optical line radiation originating in hot gas clouds are among the
signs of this activity. The activity usually originates in the centers
of these galaxies: hence the acronym AGN, for active galactic
nuclei. Species in the active galaxy ``zoo'' include Seyfert galaxies,
radio galaxies, BL Lacertae (BL Lac) objects, and QSOs. The QSO or
quasar class can be subdivided into weak radio sources and strong
radio sources (the latter group is often referred to as quasistellar
radio sources, or QSRs). Taken with more or less effort, optical
images of BL Lac objects, Seyfert galaxies, and (nearby) radio
galaxies generally show the underlying galaxy associated with the
active nucleus. Although the actual galaxy is not observed in most
quasars, all but a few astronomers nowadays classify these objects as
distant and ultraluminous AGN.
Following the selection criteria of their discoverer Carl Sevfert,
Seyfert galaxies are characterized by having small, bright nuclei
(optical) and strong emission lines in their optical spectrum. Such
emission lines are emitted by hot, ionized gas clouds, which in turn
require the presence of an intense flux of ionizing ultraviolet
photons originating in the galactic nucleus. Broad emission lines
originate in chaotic moving dense, hot gas (the broad-line region,
BLR), located within a few light-years from the nucleus of the galaxy.
Narrow emission lines are produced in a more tenuous hot gas further
out; this narrow-line region (NLR) stretches out to distances of
several thousand light-years, and, in Seyfert galaxies that are not
too far away, the NLR can sometimes be resolved with optical
telescopes. Early in the study of Seyfert galaxies, it appeared that
two subclasses in the population could be separated, based on the
relative width or strength of narrow emission lines with respect to
the broad emission lines. These lines are of comparable strength in
Seyfert type 1 galaxies, whereas the broad lines are considerably less
luminous than the narrow lines in type 2 Seyferts. Most Seyfert
galaxies produce stronger radio emission than normal galaxies;
however, the radio luminosity of a typical Seyfert galaxy is in the
range of 0.1-1% of an average radio galaxy or QSR. The radio
emission generally emanates from within the dimensions of the optical
galaxy.
Radio galaxies display strong radio emission, most of which
originates in two giant radio lobes that straddle the associated
optical galaxy. The radio sources can be of gargantuan
dimension; radio galaxy 3C 236 is the largest known object in the
universe, having a (projected!) linear size of 13 million
light-years. The associated galaxies are of (giant) elliptical type in
radio galaxies of low and moderate luminosity, but they often have
peculiar optical appearances in the most luminous cases. Most notable
in the latter cases are spatial elongations of hot emission line gas
in the direction of the extended radio emission and the presence of
morphological peculiarities such as faint tails and wisps. Luminous
radio galaxies often display strong narrow emission lines. Based on
the radio luminosity, the radio galaxy population can be divided into
into two subclasses: Above a certain luminosity the radio sources have
a linear, edge-brightened, simple double-lobed morphology, whereas
most nearby radio galaxies that are below this ``break luminosity'' tend
to have more or less complex, edge-darkened
morphologies. Whereas (two-sided, at both sides of the nucleus) radio
jets are generally observed in the latter group, the luminous radio
galaxies seldom display jets.
BL Lacertae or BL Lac objects are elliptical galaxies with very
bright, variable nuclei, displaying strong, variable radio emission,
the morphology of which is dominated by a compact radio core. Emission
lines are not seen; the redshift, and thereby distance of a BL Lac
object, is inferred from stellar absorption features in the faint
extended optical emission of the elliptical galaxy itself.
An important subpopulation of quasistellar objects (QSOs or quasars)
is formed by the quasistellar radio sources (QSRs), the radio-loud
QSOs. Historically, quasars have been characterized by a stellar
appearance and the presence of strong, broad, redshifted emission
lines. The optical spectra also display narrow, redshifted emission
lines. These redshifts, resulting from the general expansion of the
universe, indicate that quasars are the most distant objects in the
observable universe. QSRs are powerful radio sources, all exceeding
the previously mentioned break point in radio luminosity. Broadly
speaking, they have either a compact core-halo radio structure (with
dominant, variable core emission) or an extended double-lobed
morphology (with dominant lobe emission). QSRs of the latter
morphology resemble the luminous radio galaxies, although the QSRs are
smaller. A further difference with powerful radio galaxies is that
many QSRs display radio jets, but always at one side of the nucleus
only. Many of die compact QSRs vary rapidly in their optical light;
such optically violent variable (OVV) QSRs are usually combined with the
BL Lac objects into the so-called blazar class. Not all radio-quiet
QSOs are radio silent; sensitive radio telescopes detect weak, compact
radio emission in a considerable fraction of the QSO population. The
radio luminosity of these QSOs is two to four orders of magnitude
weaker than in typical, otherwise similar QSRs. The fraction of QSRs
among QSOs is about ten percent.
It will be clear that characteristic AGN properties, such as a bright
nucleus, powerful radio lobes, or strong emission lines, are shared by
members of different types. With growing databases, the notion
developed that the various AGN types could be interrelated.
The qualitative similarities between the Seyfert 1 class and the QSOs
and the quantitative continuity in their properties were already
recognized in the early 1970s. Observations of radio jets were
reported from 1977 onward, and the identification of BL Lacs with AGN
in which jets were pointing at us was subsequently suggested in 1978.
This suggestion attempted for the first time to attribute widely
differing properties of active galaxies to the effects of their
orientation. Several more were to follow, which will be discussed, in
roughly chronological order.
Using the radio astronomical very long baseline interferometry (VLBI)
technique, by the end of the 1970s three QSRs and one Seyfert type 1
galaxy had been found to display superluminal velocities in their
nuclear radio jets. The accepted explanation for this phenomenon is
based on relativistic motion in a radio jet pointing nearly at the
observer. Matter moving at nearly the speed of light in a direction
close to the line of sight will almost overtake its own radiation;
time intervals will be compressed, creating the illusion of transverse
speeds in excess of the speed of light. This relativistic beaming
model is attractive, because it also explains the apparent
one-sidedness of the radio jets, as well as the observed core
dominance in superluminal and core-halo QSRs in general, as a result
of Doppler boosting of relativistically approaching
material. Expanding on this picture, a proposal that the radio-loud
QSRs and the radio-quiet QSOs could be interrelated through
orientation, in the sense that QSRs are QSOs with jets pointing in our
direction, was put forward. This unified scheme had to be dismissed a
few years later, after close examination of the weak radio emission of
the QSOs. A subsequent proposal, however, unifying the compact,
core-halo QSRs and the extended, double-lobed QSRs through
orientation, has found rather widespread approval. Defining the source
axis as the line connecting the two radio lobes (and passing through
the nucleus), the important parameter in this scheme is the angle of
this source axis with respect to the line of sight, toward the
observer. One prediction of this scheme, namely that the lobes of a
QSR should be seen in projection on the strong, boosted core, when
seen end-on (at small angle ),
has been successful. Furthermore, the
relative numbers of core-halo and double-lobed QSRs in radio source
catalogs appear reasonably consistent with this scheme. The model
implicitly assumes that the axes of quasistellar radio sources are
randomly oriented in space. This assumption may not be valid, as we
will see later on. The alternative to this QSR unified scheme would
be the picture where intrinsically small (young?) QSRs have stronger
and more variable radio core emission. To many astronomers these and
related lines of thought appear somewhat contrived, however. A
combination of the effects of orientation and source intrinsic
effects, such as individual source evolution, is likely of course. The
relative importance of these two effects will no doubt be determined
in the next few years.
Considerable progress in understanding the interrelation between the
classes of Seyfert galaxies was made using the technique of
spectro-polarimetry, that is, spectroscopy of the polarized light
component. This technique has revealed several cases of type 2
Seyfert galaxies with obscured type l regions; the polarized light
spectrum was found to display a strong optical continuum and broad
emission lines, which are typical type 1 characteristics. The angle of
polarization in general, and the case of the prototypical Seyfert 2
galaxy NGC 1068 in particular, where nuclear light reflected off a
dust cloud was actually measured, strongly argue for the presence of
an obscuring dust torus (doughnut) around a bright continuum and broad
emission line nucleus. Seen from above or below, this nucleus is
directly visible; seen from the side, the nucleus indirectly visible
through reflection (causing the polarization) by by gas and dust above
and below the torus. Evidence for dust obscuration through
correlations with x-ray emission has also been reported, but it is not
yet clear how general these phenomena in Seyfert galaxies are.
However, there is no doubt that anisotropic radiation exists, because
an optical image of the proposed bi-conical radiation field escaping
from the presumed torus has already been obtained for one Seyfert 2
galaxy.
As described previously, the unification of compact core-halo QSRs
and extended double-lobed QSRs via the effects of jet flow speeds of
near the speed of light and orientation was quite successful in
explaining a number of observed properties. Continued VLBI monitoring
of moving radio components in QSR nuclei, however, revealed many more
cases of superluminal motion, not only in compact but also in large
double-lobed QSRs. Using sophisticated radio imaging techniques, it
was found that radio jets in QSRs generally occur at one side of the
nucleus only. In the framework of relativistic beaming, this would
imply that many if if not all QSRs are oriented more or less toward
us. Stated other wise: QSRs whose relativistic beams are
perpendicular to our line of sight in effect hide or masquerade
themselves as others sorts of objects. Based on the facts that
powerful radio galaxies exist everywhere in the (history of the)
universe where QSRs exist, and that the former are on average a factor
of 2 larger in projected size and have comparable narrow emission line
luminosities (hence comparable level of activity in the NLR),
unification of these objects was proposed. As in the Seyfert case, a
dusty torus perpendicular to the radio axis could hide the QSR broad
emission line region as well as the bright continuum radiation, when
observed from the side (edge-on). See
Fig. 1. Such a configuration would would
naturally explain the extension of the emission line gas as discovered
in several powerful distant radio galaxies, as due to cones of of
ionizing radiation emitted by a hidden energy source and escaping
along the radio axis. Also, the strong excesses as fear-infrared
wavelengths reported in these radio galaxies could well be due to
reradiation by obscuring dust. Many observational facts are consistent
with this unified model, but the case has to be fully established.
The alternative picture would be to identify radio galaxy with a
burned-out QSR. A QSR in which the violent nuclear activity has
(temporarily) stopped would look like a powerful radio galaxy, but the
questions as to the existence of truly randomly oriented QSRs or the
origin of the apparent radio morphological asymmetries remain. As
mentioned already, the relative importance of orientation and object
evolution will be a subject of detailed study in the coming years.
Figure 1. Radio galaxies, quasisrellar
radio sources, and blazars may be the same objects viewed from
different directions.
Although additional effects of orientation are not ruled out, the
class of radio-quiet QSOs is most likely evolutionary linked to
another, recently recognized, class of active galaxies, namely
powerful infrared galaxies. These objects were discovered by the
infrared astronomy satellite lRAS and produce more than 1012
L in the
infrared part of the electromagnetic spectrum. These luminosities are
comparable to QSO luminosities. Because these infrared galaxies also
display QSO spectral characteristics and their optical morphologies
resemble those of some nearby QSOs, identification with
dust-enshrouded QSOs was proposed. Once all the dust has been blasted
away, a brilliant QSO should appear. Equally important is the fact
that optical images of these QSOs-in-the-making suggest galaxy
interaction as the ultimate origin of the nuclear activity.
In a way that is complementary to the unification of radio-loud QSRs
with powerful radio galaxies, the association of BL Lac objects with
favorably oriented radio galaxies of low and moderate luminosity has
been suggested. BL Lac objects occur rather close to our
galaxy. Unifying them with the powerful radio galaxies which are rare
in the local universe is therefore not possible. Lower luminosity
radio galaxies do exist in larger numbers locally, and because their
radio lobe luminosities are comparable to the luminosity of the halo
emission in BL Lac objects, unification of these classes of objects is
likely. The absence of emission lines in BL Lacs is no surprise in
this picture, because the lower luminosity radio galaxies also lack
those lines. Moreover, the optical emission of BL Lac objects is
strongly dominated by the (polarized) non-thermal jet component. Note
that the OVV QSRs, which together with the BL Lacs make up the blazar
class, do have emission lines. Compared to other QSRs, the effects of
beaming are most pronounced in OVV QSRs, which are therefore regarded
as QSRs with jets closest to the sight line.
For the previous interrelating schemes to hold, it is imperative that
the properties of the host galaxies as well as their local
environments are undistinguishable. Investigations to test the various
predictions concerning host galaxies and environments are currently in
progress.
Important steps have been made in the past 10 years to explain the
observed AGN diversity as a result of the combined effects of beaming,
projection, and anisotropic radiation/obscuration in a small number of
intrinsically different, evolving classes of objects. The effects of
orientation and dust obscuration appear far more important than
previously thought. The puzzle is not yet solved, but many pieces of
the picture seem to be falling into place.
ACTIVE GALAXIES AND QUASISTELLAR OBJECTS, INTERRELATIONS OF VARIOUS
TYPES
Antonucci, R. (1989). Evidence for and against relativistic beaming in
active galactic nuclei. Proceedings 14th, Texas Symposium on
Relativistic Astrophysics. New York Academy of Sciences, New York.
Barthel, P. D. (1989). Is every quasar beamed? Ap. J. 336 606.
Blandford, R. D., Begelman, M. C., and Rees, M. J. (1982). Cosmic Jets.
Scientific American 246 (No. 5) 84.
Sanders, D. B., Scoville, N. Z., and Soiler, B. T. (1988). IRAS 14348-1447,
an ultraluminous pair of colliding, gas-rich galaxies: The birth of a
quasar? Science 239 625.
Zensus, J. A. and Pearson, T. J. , eds. (1987). Superluminal Radio
sources. Cambridge University Press, Cambridge.
See also Active Galaxies and Quasistellar Objects, Jets; Active
Galaxies and Quasistellar Objects, Superluminal Motion; Galaxies, Radio
Emission.