Annu. Rev. Astron. Astrophys. 1980. 18: 165-218
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2.2. Taxonomy of Extended Sources

The taxonomic approach to radio morphology attempts to recognize recurrent patterns that could give useful phenomenological information about the mechanisms involved. Pattern recognition depends on the amount of detail delineated by a given observation, i.e. both on the number of pixels (beam elements) across the source and on the dynamic range in brightness that can be studied.

Bearing these remarks in mind we now consider some similarities and differences between the various types of radio sources described in Section 2.1. The duplicity of the large-scale structure, the widespread presence of radio cores, and the nuclear collimation in wide sources (see Section 4.3) all suggest that the mechanisms for producing the radio sources are similar and occur in the parent galaxy nuclei.

However, as we have seen, there are also several differences. Three gross structural properties have emerged as being particularly important in studying these morphological differences. In order of the increasing amount of relative detail needed to discern them they are 1, the overall bending at the outer extremities, 2. the edge-brightening, 3, rotational symmetry. These properties define three sequences along which the large-scale structure of almost all sources can be classified, allowing for projection effects.

2.2.1 BENDING SEQUENCE RADIO TRAILS The bending sequence is illustrated in Figure 6. The simplest measure of source bending is the angle subtended at the radio core or nucleus by lines drawn to the extremities of opposite lobes. An analysis of the distribution of this "opening angle," chi, for a sample of 44 tailed and double sources has been given by Valentijn (1979b). Opening angles between 0° (narrow-tailed source) and 180° (double source) are seen, with a possible small zone of avoidance 45° < chi < 90°. The bending sequence extends fairly continuously from double sources to narrow tailed sources, so any discussion of bending that includes only double sources (Harris 1974, Ingham & Morrison 1975) seems unnecessarily restrictive. The opening angle seems to be correlated with the absolute magnitude of the parent galaxy; narrow tails are associated with fainter galaxies than the wider tails (Rudnick & Owen 1977, McHardy 1979, Simon 1978, Valentijn 1979b). The bending is most simply explained as distortion of an initially collimated double morphology by translational motion with respect to a surrounding intergalactic medium (Miley et al. 1972).

Figure 6

Figure 6. A sequence of radio-source bending illustrated with schematic drawings of actual sources.

Within this picture, therefore, tailed sources are viewed as trails or fossil records deposited by active galaxies. This interpretation is supported by (a) similarities between the fine-scale structure of tailed and double sources (Section 4.4; Miley 1974) and (b) the preferential occurrence of the most bent sources in dynamically active clusters containing hot gas (Section 5.4). A number of radio-trail models have been proposed to account for the detailed brightness and polarization distributions (e.g. Jaffe & Perola 1973, Pacholczyk & Scott 1976, Cowie & McKee 1975, Jones & Owen 1979).

2.2.2 EDGE-BRIGHTENING SEQUENCE - MORPHOLOGY AND LUMINOSITY The edge-brightening sequence is illustrated in Figure 7. There is a striking correlation between the degree of edge brightness of a source and its luminosity. This was first noted by Fanaroff & Riley (1974) who examined the structure of 3C sources as a function of luminosity at 178 MHz (P178). For P178 ltapprox 1026 W Hz-1 ("Class I") almost all sources have a relatively relaxed morphology, while for P178 gtapprox 1026 W Hz-1 ("Class II") almost all are edge-brightened doubles.

Figure 7

Figure 7. Sequences of edge-brightness and rotational symmetry illustrated with schematic drawings of actual sources.

This critical luminosity is tantalizingly close to the break in the radio luminosity function for elliptical galaxies, which, scaling from Auriemma et al. (1977), occurs at ~ 1.9×1025 W Hz-1. The break in the luminosity function separates sources that show strong population evolution (the more luminous sources) from those that do not.

Further work (Jenkins & McEllin 1977, Speed & Warwick 1978) suggested that for the edge-brightened doubles themselves the fractional flux in the hot spots increases with luminosity. However, Kapahi has pointed out that this might be caused by selection. Owing to the limited resolution on the maps of the strong sources, linear size evolution of the hot spots might cause the more distant (stronger) ones to appear more intense.

The lower luminosity (Class I) sources are a mixed bag. They include wide doubles, edge-darkened doubles, tailed, and complex sources. There are indications that the degree of bending (the opening angle in the bending sequence) decreases with luminosity, the wide-angle tails being more luminous than the narrow-angle tails (Owen & Rudnick 1976, Simon 1978, Valentijn 1979b). This can be qualitatively understood; One would expect outbursts in the more powerful sources to involve more kinetic energy, and the distorting pressure of an intergalactic medium on their evolution to be consequently smaller. Alternatively, the lower luminosity sources are presumably associated with less massive galaxies which may on average be moving faster.

2.2.3 ROTATIONAL SYMMETRY Also shown in Figure 7 are some examples of two-dimensional rotational symmetry (S or Z shape) which is seen in many radio sources (Harris 1974, Miley 1976). This symmetry was first observed in Centaurus A (Cooper et al. 1965) and is particularly evident in 3C47 (Pooley & Henbest 1974, Miley & Hartsuijker 1978), 3C272.1 (Jenkins et al. 1977), NGC 315 (Bridle et al. 1976, Bridle et al. 1979a), and the outer components of Cygnus A (Hargrave & Ryle 1974). A possible cause is distortion of the structure by rotation or shear in the intergalactic medium. But to explain the Z shapes in giant sources such as NGC 315 such motions must take place over, scales of more than a megaparsec. An alternative hypothesis is a swinging of the fundamental nuclear ejection axis during the lifetime of the source (Miley 1976). The symmetries in 3C315 (Northover 1976, Högbom 1979) and B2 0055 + 26/4C 26.03/NGC 326/ (Ekers et al. 1978a) are indeed suggestive of precessing nuclear beams. A scenario in which such precession could occur was sketched by Rees (1978b). Misalignment between the rotational axis of the nuclear machine (a black hole) and the angular momentum axis of the galaxy is supposed to take place as a result of galaxy merging. It is noteworthy that both 3C315 and B2 0055 + 26 are associated with close pairs of galaxies. Precession could also occur as the central black hole was accreting material from a cloud or disk with a different angular momentum direction from that of the galaxy as a whole. See Section 5.3.

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