Annu. Rev. Astron. Astrophys. 1980. 18: 165-218 Copyright © 1980 by Annual Reviews. All rights reserved

2. BASIC RADIO SOURCE STRUCTURE

Not chaos-like, together crushed and bruised,
But as the world harmoniously confused;
Where order in variety we see,
And where all things differ, all agree.

Alexander Pope, "Windsor Forest"

2.1. Commonly Observed Morphologies

We shall first describe the classes of radio structure that are commonly seen and then show how these morphologies can be ordered in various sequences.

2.1.1 NARROW EDGE-BRIGHTENED DOUBLE SOURCES These sources, which are variously referred to as "classic doubles," "symmetric doubles," "Cygnus A-type doubles," "Type I doubles," and "triples," form the major constituent (~ 70%) of low frequency radio surveys at high flux levels (3C, 4C, Parkes). They have length-to-width ratios 4, and lobes that are brightened at their edges and symmetrically straddle an elliptical galaxy or QSO. Most radio galaxies with 178-MHz luminosities 10²⁵ W Hz^-1 and most extended radio sources associated with quasars are of this type.

When studied at high resolution ( 5 pixels across the source) the edge brightening is usually seen to be due to intense compact regions of emission or "hot spots" at the outer edges of the radio lobes. The hot spots have sizes that are typically ~ 1 kpc (Kapahi 1978), i.e. two orders of magnitude smaller than the source as a whole (see Section 2.3). Extended tails or bridges often reach from the hot spots towards the associated galaxy or QSO.

A flat-spectrum radio core is usually associated with the nucleus of the parent galaxy or QSO (see Section 4.3). For this reason these sources are sometimes called "triple." The example shown in Figure 1 is 3C452 (Högbom 1979).

The overall structure of edge-brightened doubles has several important morphological properties.

(a) Collinearity As can be seen from Figure 1 the hot spots line up accurately with the nuclear radio core. This is a common property of edge-brightened doubles (e.g. Hargrave & McEllin 1975). In Cygnus A the hot spots and the core are collinear to better than 1^o (Miley & Wade 1971, Hargrave & Ryle 1976).

Figure 1. 3C452, a typical narrow edge-brightened double source. Westerbork 5 GHz intensity contour map with magnetic field directions superimposed. The cross indicates the position of the nucleus of the associated galaxy (from Högbom 1979 and kindly supplied by the author).

(b) Overall symmetry The distributions of flux ratios and positional asymmetries between opposite lobes of edge-brightened double sources have been studied at 1.4 GHz (Fomalont 1969, Mackay 1971) and at 5 GHz (Riley & Jenkins 1977, Katgert-Merkelijn et al. 1980, Longair & Riley 1979). The reader should be cautious in reading too much into the detailed statistics because the likelihood of an optical identification is biased in favor of symmetric sources. It is clear, however, that components of limb-brightened doubles are quite similar. The strongest component is observed to be more than twice as strong as the weaker one in only about 30% of the sources. The components are also usually symmetrically distributed with respect to their parent nuclei; the distances to opposite extremities rarely differ by more than 50%. Components of sources associated with quasars tend to have less similar flux ratios and to be more asymmetric than those associated with galaxies. For the radio galaxies the components closest to the nuclei tend to be the brightest, but this trend is not apparent in the case of the quasars.

In some sources (e.g. 3C273; Conway & Stannard 1975), extended structure is seen only on one side of the nucleus, the flux ratio between opposite lobes usually exceeding 10:1. These highly asymmetric sources are sometimes called "D2 doubles" (Miley 1971, Conway & Stannard 1975, Davis et al. 1977, 1978) or "C-sources" (Readhead et al. 1978a). It is not clear whether these sources are members of a different species or whether they represent the tail of a continuous distribution of relative brightness. Three of them have recently been mapped with the VLA (Perley & Johnston 1979). Such sources make up ~ 10% of all extended quasars (Stannard & Neal 1977, Miley & Hartsuijker 1978). One-sided sources identified with galaxies are rarer but are still occasionally seen (e.g. Argue et al. 1978). Apparent differences between radio galaxies and quasars may merely reflect a dependence on luminosity, since the more luminous sources are almost all quasars; see Section 5.2.

(c) Alignment The agreement between the orientation of the cores and that of the overall structure of symmetric edge-brightened doubles is discussed in Section 4.3.3. Alignment on an intermediate scale is seen in the bridges that extend from the hot spots to the cores and in which enhancements are sometimes present (e.g. Goss et al. 1977).

2.1.2 NARROW EDGE-DARKENED DOUBLE SOURCES Several narrow double sources have brightness distributions that gradually die away at their extremities. The closest radio galaxy Centaurus A (Cooper et al. 1965) falls into this class. Figure 2 shows 3C 449, another good example (Perley et al. 1979, Bystedt & Högbom 1979). 3C31 (Burch 1977b) and the two radio galaxies that make up 3C 402 (Riley & Pooley 1975) are also of this type.

Figure 2. 3C449, a narrow edge-darkened double source. Westerbork 1.4 GHz intensity contour map (from J. Bystedt and J.A. Högbom, in preparation, and kindly supplied by the authors).

These radio sources usually have radio cores (Section 4.3). When studied with high enough resolution, narrow collimated radio jets are often seen to emanate from their nuclei (Section 4.4). They are sometimes called "3C31-types" (Simon 1978).

2.1.3 WIDE DOUBLE SOURCES A few relatively relaxed doubles have fatter lobes with length-to-width ratios of 2 or 3. Figure 3 shows 3C310 (van Breugel 1980a). Other examples are Virgo A (Andernach et al. 1979), Fornax A (Cameron 1971), Centaurus A (Cooper et al. 1965), and 3C314.1 (Miley & van der Laan 1973).

Figure 3. 3C 310, a wide double source. Radiophoto of the total intensity distribution (Westerbork 5 GHz) with magnetic field directions superimposed (from van Breugel 1980a and kindly supplied by the author).

2.1.4 NARROW TAILED SOURCES These have a tadpole-like asymmetric structure, with a high brightness head that coincides with the nucleus of an elliptical galaxy and a diffuse tail that often extends for several hundred kpc. Alternative names by which this species is known are "head tail" or "narrow angle tail" (NAT) source. In some cases their morphology is observed to be two-pronged with a double tail and/or a double head. The ability to distinguish between "twin tailed" and "single tail" sources sometimes depends on the resolution and dynamic range of the observations.

Shown in Figure 4 is 3C83.1B/NGC 1265 (twin) and in Figure 6 IC 310 (single), both radio galaxies in the Perseus Cluster. Other examples of narrow twin-tailed sources are 3C129 (Miley 1973), B2 1108 + 41 (Rudnick & Owen 1977, Valentijn 1979a), B2 1502 + 28 (Valentijn 1979a), and 4C 39.49 (Miley & Harris 1977).

Figure 4. 3C 83.1 B/NGC 1265, a twin-tailed source in the Perseus Cluster. Radiophoto of the total intensity distribution (Westerbork, 1.4 GHz) with magnetic field directions superimposed (from Miley et al. 1975).

Radio cores are usually found in tailed sources (see Section 4.3). When studied with sufficiently high resolution the heads are often resolved into narrow jets of radio emission (see Section 4.4).

2.1.5 WIDE-TAILED SOURCES The structure of these sources is intermediate between that of the narrow edge-darkened doubles and the narrow twin-tailed sources. A common alias by which they are known is "WAT" (wide-angle tail). Shown here, in Figure 5, is 3C465 (van Breugel 1980c). Other examples are IC 708 (Vallée & Wilson 1976, Wilson & Vallée 1977, Vallée et al. 1979), B2 0658 + 33B, B2 0836 + 29, and B2 0838 + 32A (Valentijn 1979a), NGC 6034 (Valentijn & Perola 1978), and 4C 48.21 (Owen & Rudnick 1976).

Figure 5. 3C465/NGC 7720, a wide-angle tailed source. Radiophoto of the total intensity distribution with magnetic field directions superimposed (from van Breugel 1980c and kindly supplied by the author).

2.1.6 CLUSTER HALOS Several clusters of galaxies have associated large radio components with steep nonthermal spectra ( -1.2) which often dominate the cluster radio emission at frequencies below 100 MHz (e.g. Baldwin & Scott 1973, Slingo 1974). Instruments operating at this low frequency have insufficient resolution to map them and they are extremely difficult to detect at high frequencies owing to their faintness and confusion from other sources in the cluster. Therefore little is known about them. The sources of this class to have been studied in most detail are Coma C, a smooth diffuse halo in the Coma Cluster with a size of ~ 40' (1.2 Mpc; Jaffe et al. 1976, Jaffe 1977, Valentijn 1978, Hanisch et al. 1979) and Abell 2256 where the situation is complicated (Bridle & Fomalont 1976, Masson & Mayer 1978, Bridle et al. 1979b). Other possible examples are in Abell 1367 (Gavazzi 1978) and Abell 2139 (Harris & Miley 1978), but the cluster halo 3C84 B reported to be in the Perseus Cluster (Ryle & Windram 1968) has been shown to be nonexistent (Gisler & Miley 1979, Birkinshaw 1980, Jaffe & Rudnick 1979).

It has been suggested that cluster halos are the remnants of tailed radio galaxies whose spectra are steepened by synchrotron losses (e.g. Harris & Miley 1978) or interfaces between two subclusters undergoing coalescence (Harris et al. 1980).