13.4.1. Jets, Lobes, and Hot Spots
The apparently simple properties of powerful, (i.e., L > 1040 erg s-1) extragalactic radio sources have been complicated by the high-resolution maps which have become available over the past fifteen years with ever increasing detail. However, at the same time, the nature of the physical processes necessary to explain these sources have become clearer.
For some time it has been clear that a general description of an "extragalactic source" includes a central component and some sort of extended double structure. During the 1980s, it has become evident that virtually all such sources also have narrow elongated tubes of radio emission connecting the central source to the outlying extended structure, suggesting that energy, magnetic field, relativistic particles, and probably thermal gas are being transferred away from the nucleus to form the observed extended structures.
The detailed morphology illustrated in Figure 13.10 has resulted in an updated form of the Rees (1971) beam model being generally accepted as the working picture of extragalactic radio sources. The models of Rees envisioned an invisible, relativistic flow which terminated in a shock where the flow energy was randomized and emerged, at least partly, as the flux of relativistic particles necessary to maintain the extended emission. Unlike the original model, however, the radio jets now commonly observed, emit radiation. Thus, the transport process must not be entirely efficient. Also many jets are observed to bend and wiggle, probably indicating an interaction with an external medium. In nearby sources, this medium is often observed through its X-ray emission and appears to be in pressure equilibrium with the plasma in the jet. Properties of radio jets are discussed more fully by Bridle and Perley (1984).
Figure 13.10. (a) Cygnus A is the strongest extragalactic radio source in the sky and is the prototypical example of FR If radio structure. The image of the extended structure was made using the VLA (From Perley et al. 1984) and shows a faint radio jet which apparently feeds the outer hot spot on one side and the filamentary radio lobes. The structure shown for the compact core is based on VLBI data (From Downes et al. 1981). (b) 3C334 has a curved knotty jet emanating from the bright unresolved source that coincides with the quasar. Both lobes contain some filamentary substructure. The thin filament extending back toward the quasar from the northwestern lobe may be part of a weak counterjet, which is rarely seen in powerful sources except on images of very high dynamic range. Counterjets are very common, however, in low-power sources. (Observers: Owen and Hines. Courtesy NRAO/AUI.) (c) M87 is a relatively weak radio galaxy in the Virgo cluster. The jet, shown here, is about 2 kpc in extent and its emission extends from the radio into the optical and X-ray regions of the spectrum. (d) The unresolved bright spot near the center of this 5GHz image is coincident with a quasar at a red shift of 0.77. The long, narrow, one sided radio jet is typical of powerful double lobed sources. There are prominent hot spots in both lobes suggesting that they have both been recently supplied with relativistic particles despite the appearance of only a single jet. (c and d: Observers: A. Bridle, I. Browne, J. Burns, J. Dreher, D. Hough, R. Laing, C. Lonsdale, P. Scheuer, J. Wardle).
Extended extragalactic sources also show a basic change in their morphology at an absolute luminosity of P(14 GHz) ~ 1025W Hz-1. Sources weaker than this level appear limb darkened, that is, they slowly fade away in brightness as one looks further away from the nucleus, while brighter sources have limb-brightened outer structures. The two classes are referred to as Fanaroff and Riley (1974) classes I and II, respectively (or FR I and FR II). FR II sources also often show small "hot spots" either at the farthest edges of the source or sometimes apparently embedded in more diffuse structure. It is believed that the entire limb-brightened structure is due to supersonic jets terminating at a boundary with an external medium surrounding the source. FR I sources, on the other hand, may be subsonic, at least in their outermost regions. They are thought to be in thermal pressure balance with the external medium and possibly to have entrained a great deal of external gas. FR I sources are often found in nearby rich clusters of galaxies and are often distorted by processes in the clusters.
Because of their higher luminosity and relatively low space density, FR II sources tend to be identified with distant radio galaxies and quasars, often near the edge of the observable universe. However, some examples do exist relatively nearby, such as Cygnus A which has a redshift of 0.057. Its parent galaxy has long been known to have very strong emission lines. The galaxy is also very bright at optical wavelengths but unfortunately lies within ten degrees of the galactic plane, which makes it difficult to study optically. It lies in the center of a very dense (ne ~ 10-2 cm-3) cloud of very hot (108 K) gas. Lower-luminosity examples of the class, which are more common nearby, do not generally show such extreme X-ray properties and, except for some nuclear emission lines, resemble normal giant elliptical galaxies.
The dominant morphological characteristic of FR II radio galaxy emission is the brighter outer lobes. Often embedded in the lobes are more compact hot spots, perhaps as small as one kiloparsec. This structure is usually accompanied by a compact core in the center of the galaxy, although the core is sometimes too weak to be seen with present maps. Finally, in nearby examples which have been studied very extensively with the VLA, a faint jet can be seen connecting the nucleus with the outer hot spots and lobes, at least on one side of the double. Such jets have been seen in only a few cases, and it is unclear whether the jets are strongly one-sided as in the quasars discussed below.
The general properties of this morphology have recently been convincingly reproduced in numerical simulations of low-density, supersonic jets traveling through a denser external medium. This type of work is just beginning but suggests that we are on the right track and may ultimately be able to understand a great deal about these sources.
Quasars also produce FR II sources. However, the relative importance of the distinct features of the general morphology often is different. The bright outer lobes and hot spots are still visible but the central component and the jets are much brighter, both relative to the lobes and in absolute terms. The luminosity of the central component and jet of quasars is usually one to two orders of magnitude higher than for the galaxies. Quasar jets almost always appear to be one-sided. Furthermore, these jets often show many bends and wiggles, sometimes by as much as ninety degrees. They often seem to be made up of many small knots rather than a continuous brightness distribution.
The origin of the one-sidedness is, at present, still unclear. On the one hand, many of the properties seem consistent with relativistic beaming in the line of sight, as is used to explain the observed compact jets (Section 13.3.7). The ratio of brightness from one side to the other in any given case is consistent with fairly small values of 's. Also the pronounced wiggles and bends can be more easily understood if they are intrinsically small wiggles, which when inclined to the line of sight, appear to be large bends in projection on the sky. Moreover, where VLBI and VLA observations exist for the same source, the one-sided jets on both scales lie on the same side of the source, suggesting a common origin for the observed one-sidedness.
However, it is hard to understand how this can be the case since in radio quasars we see only one-sided jets and we almost always can detect a jet. Also, quasars with one-sided jets and bright radio cores exist which appear as large as all but a few of the largest radio galaxies, so they are unlikely to be appreciably foreshortened (e.g., Schilizzi and de Bruyn 1983). Alternatively, the high-luminosity jets may be intrinsically one-sided. Since the diffuse lobe emission is usually found on both sides of the nucleus, this seems to imply that either the missing jet is not radiating as strongly for some reason or that the jet "flip-flops" between the two sides (Rudnick and Edgar 1984). Also, since there is strong evidence for relativistic motion in the core (Section 13.3.7), such a picture implies that the jet slows down a great deal on its journey to the outer lobe. Neither picture is entirely satisfactory at this time.
Both FR I and FR II sources can exhibit large degrees of linear polarization, 50% locally; however, the jets in the two types of sources usually show very different field geometry. Most straight FR I sources show either magnetic fields predominantly perpendicular to the jet axis or perpendicular fields which change to predominantly parallel fields at some point down the jet. Exceptions to this trend sometimes occur in very bent sources, where stretching and shearing near the bend may cause an apparent parallel-to-perpendicular flip in the magnetic field. In FR II sources, the magnetic field is usually parallel to the jet axis all the way along the radio jet. The lobes of FR II sources, however, usually have the magnetic field running along the outer edge of the lobe.
The lower-luminosity FR I sources which have been studied up to now are much more nearby on average, and thus we know more about the environment in which they exist. Virtually all are found in some sort of galaxy clustering from poor groups up to the richest clusters. In many of these cases, especially in rich clusters, we know from X-ray observations that they are surrounded by a hot (107 to 108 K), relatively dense (10-2 to 10-4 cm-3) medium. The pressures inside the radio sources implied by minimum-energy calculations are often equal to or less than the pressure of the external hot medium. This relationship plus the relaxed-looking, distorted nature of FR I sources suggests that the interaction with the external medium is extremely important in determining the properties of these sources.
FR I sources take on a variety of morphological shapes. However, some general patterns can be recognized. The most luminous FR I sources are usually associated with bright D or cD galaxies located in the center of their associated cluster. They are usually one or two magnitudes brighter than the giant elliptical galaxies usually associated with (nearby) FR II sources. These galaxies also usually have a less rapidly dropping light distribution, suggesting a flatter gravitational potential of much larger extent than is found for the FR II sources. Their radio morphology can usually be described as a twin jet with a gradually widening and often bending channel. Many of these sources, especially those in rich clusters, are bent into C-shapes. These sources are called wide-angle tails.
At lower luminosities in rich clusters, one often finds sources which have apparently been even more distorted with jets which have been bent by ninety degrees on each side of the galaxy and merge into long diffuse tails. These sources are called narrow-angle tails. Their parent galaxies are intermediate in optical luminosity between wide-angle tails galaxies and FR II sources. They are believed to be formed by a normal radio galaxy moving through the hot, tenuous medium in a cluster (e.g., Owen et al. 1979), although this may be hard to reconcile with the very longest-tailed sources (Burns 1981).
Almost all FR I sources have a prominent central component, although this is mainly due to the lower surface brightness of the extended emission compared with FR II sources. Most show two-sided jets. These jets are often close to being equal in brightness, and in a few cases the orientation of dust lanes actually suggests that the slightly brighter jet is pointed away from us. Thus, relativistic motion is not indicated for these sources, at least far away from the nucleus.