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 theta 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 theta), 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

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 Lsmsun 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.

Additional Reading
  1. 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.
  2. Barthel, P. D. (1989). Is every quasar beamed? Ap. J. 336 606.
  3. Blandford, R. D., Begelman, M. C., and Rees, M. J. (1982). Cosmic Jets. Scientific American 246 (No. 5) 84.
  4. 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.
  5. Zensus, J. A. and Pearson, T. J. , eds. (1987). Superluminal Radio sources. Cambridge University Press, Cambridge.
  6. See also Active Galaxies and Quasistellar Objects, Jets; Active Galaxies and Quasistellar Objects, Superluminal Motion; Galaxies, Radio Emission.