|Annu. Rev. Astron. Astrophys. 1980. 18:
Copyright © 1980 by . All rights reserved
The fundamental cause of the development of a thing is not external but internal; it lies in the degree of contradiction within the thing.
Mao tse Tung. "On Contradiction"
4.1. Diffuse Emission
Most radio sources are associated with some diffuse emission. The low-brightness extremities of tailed sources and narrow relaxed doubles have similar diffuse appearances, and the wide doubles have relaxed amorphous lobes. In edge-brightened doubles the diffuse emission takes the form of low-brightness bridges reaching from the outer hot spots towards the nuclei, but the distinction between bridges and radio jets (Section 4.4) is not always clear.
4.1.1 PROPERTIES The diffuse emission is characterized by a relatively steep spectrum (- 0.7 -1.2). For tails the spectrum is sometimes observed to steepen with distance from the parent galaxy, indicating that radiative losses of the type treated in Section 3.1.4 may be occurring (e.g. Miley 1974, Schilizzi & Ekers 1975, Valentijn & Perola 1978). Similar behavior has been observed in the edge-darkened double 3C 31. However, the form of the spectral steepening in this source, one of the few that has been mapped at more than two frequencies, is not that expected from simple synchrotron losses (Burch 1977b). In contrast to the spectral variations observed in the low luminosity sources, spectral steepening has been reported in an inward direction, away from the outer edges of some edge-brightened doubles (Burch 1977a, b, 1979a, b, Dreher 1979, Högbom 1979) but not others (Jenkins & Scheuer 1976, Gopal-Krishna 1977, Gopal-Krishna & Swarup 1977).
Despite the spectral steepening that is sometimes observed, application of the arguments in Section 3.1.4, with the equipartition proviso, suggests that localized particle acceleration occurs frequently in tails (e.g. Wilson & Vallée 1977, Hintzen et al. 1977, Ekers et al. 1978b, Baggio et al. 1978, Simon 1979, Downes 1980) and also throughout the lobes of some double sources (Burch 1977b, Willis & Strom 1978, Strom & Willis 1979, Burch 1979b, van Breugel 1980a).
Fine structure is often seen in the bridges of edge-brightened doubles These enhancements may correspond to periods of increased activity in the nuclear history, or pinpoint regions where localized particle acceleration is occurring. High sensitivity measurements of the wide double 3C310 have also revealed weak fine structure on a scale of a few kiloparsecs within its diffuse lobes (van Breugel 1980a).
The diffuse emission is usually highly polarized with polarizations often approaching 60% at frequencies above 1 GHz indicating the presence of a well-ordered magnetic field (Section 3.2). The field direction is generally observed to be circumferential (e.g. Fomalont 1972, Miley & van der Laan 1973, Strom et al. 1978, Willis & Strom 1978, Andernach et al. 1979, Burch 1979b, De Young et al. 1979, van Breugel 1980a, Strom & Willis 1979). See also Figures 1, 3, and 5. For the diffuse bridges and tails the known polarization directions therefore indicate average fields directed along the components (e.g. Miley 1976, Miley et al. 1975, Högbom 1979, Burch 1979b), although locally the magnetic field directions are often more complex, particularly near regions of enhancement in total intensity.
4.1.2 CONFINEMENT AND SHAPING How are the diffuse emitters held together? Evidence that the confining agent is thermal pressure of a hot gas comes from X-ray measurements (e.g. Gursky & Schwartz 1977, Mushotzky et al. 1978), which show that, at least in some of the rich clusters that house radio sources with diffuse lobes, there is an intergalactic medium with ~ 10-27.5 g cm-3 and T ~ 107.5 K. By the arguments of Section 3.1.2 the pressure of such a gas would be sufficient to confine the lobes provided their internal energy is not much greater than the minimum permitted values (Bme ~ 10-5.5 G). If thermal pressure does indeed confine tails, the discovery of two tailed galaxies in poor clusters (Ekers et al. 1978b) suggests either that an intergalactic density of ~ 10-27.5 gm cm-3 may be widespread in the Universe or, as is perhaps more likely, that the sources are confined by giant circumgalactic gaseous halos with dimensions comparable with or larger than the extended radio emission (Norman & Silk 1979).
One of the properties of the diffuse emission that has received considerable attention is the curvature frequently occurring in the tails of tailed radio galaxies. Significantly curved tails have been observed in 3C83.1B, 3C129 (Miley et al. 1972, Miley 1973), IC 711 (Vallée & Wilson 1976, Wilson & Vallée 1977), 5C 4.81 (Jaffe et al. 1976), 1200 + 519/4CT 51.29.1, and 1709 + 397/4CT 39.49.01 (Miley & Harris 1977). Several explanations have been proposed for this curvature - all within the context of the radio-trail model (Section 2.2.1). It has been suggested that the tail traces a curved orbit of the parent galaxy in the gravitational field of the cluster (Miley et al. 1972) or of a neighbouring galaxy (Byrd & Valtonen 1978). Alternatively, the tail may be distorted by large-scale shear in the velocity field of the intergalactic gas (Jaffe & Perola 1973) or by buoyancy forces in the gravitational field of the cluster (Cowie & McKee 1975). Probably more than one of these mechanisms contributes to bending radio tails. Interpretation of the observations is complicated by effects of projection.
Buoyancy is a particularly interesting possibility. It would cause tails lighter than their surroundings to rise away from the cluster centers. According to Cowie & McKee (1975) denser "detached plasmoids" may be left behind in this process. This may be occurring in the case of 3C 83.1B/NGC 1265 in the Perseus Cluster whose low brightness structure is shown in Figure 8. It has been suggested (Gisler & Miley 1979) that the weak extension to the northeast is such a "heavy tail," which traces the actual orbit of NGC 1265, while the main body of the tail has been bent away from the cluster center by buoyancy forces. On this basis, making several additional assumptions about the geometry, one obtains a lower limit to the mass of the Perseus Cluster of ~ 1015 M, similar to that obtained independently from the virial theorem. There are also indications that buoyancy plays a part in shaping the structures of diffuse emission in double radio galaxies, e.g. 3C264, 3C315 (Northover 1976), and 3C449 (Bystedt & Högbom 1979).
Figure 8. 3C83.1B/NGC 1265, the outer contours are from Westerbork 0.6-GHz intensity measurements and the inner contours were taken from higher resolution 1.4-GHz data (from Gisler & Miley 1979). The cross shows the position of the nucleus of NGC 1265 and the arrows indicate the direction to the center of the Perseus Cluster. The weak northeastern extension may represent the trajectory of the galaxy and the main tailed structure a buoyant tail floating away from the center of the Perseus Cluster.