Narrowangle Tailed Radio Galaxies

The most striking example of the interaction between the ICM and radio sources is provided by the head-tail, or NAT, sources (Rudnick & Owen 1976, O'Dea & Owen 1985). These sources have radio jets that are bent at extreme angles, up to 90°, from their original orientation. They have typical luminosities of FRI sources, and are identified with cluster galaxies located at any distance from the cluster center. The NAT radio sources generally show a high degree or polarization in the tails (up to 40-50%) with the intrinsic magnetic field parallel to the tail direction. Typical ages at the end of the tails, derived from spectral index arguments, are around 5 107 years (e.g., Feretti et al. 1998, 1999).

Figure 6.4• Radio image at 5 GHz of the Narrow Angle Tailed radio source NGC1265 (contours), superimposed on the blue Palomar Sky Survey print (grey-scale). The radio nucleus coincides with the optical galaxy. The two jets are bent backward and form low brightness tails in a common direction. The image is courtesy of Wellington et al. (1973).

The prototypical example of a NAT radio source is NGC1265 in the Perseus cluster (Wellington et al. 1973, see Fig. 6.4). The standard interpretation for this radio morphology is that the jets are curved by

Jet Morphologies Radio Contours

Figure 6.4• Radio image at 5 GHz of the Narrow Angle Tailed radio source NGC1265 (contours), superimposed on the blue Palomar Sky Survey print (grey-scale). The radio nucleus coincides with the optical galaxy. The two jets are bent backward and form low brightness tails in a common direction. The image is courtesy of Wellington et al. (1973).

ram pressure from the high-velocity host galaxy moving through the dense ICM (Miley et al. 1972), whereas the low brightness tails are material left behind by the galaxy's motion. The ram pressure model was first developed by Begelman et al. (1979), and studied in further detail by Vallée et al. (1981) and Baan & McKee (1985). Following dynamical arguments, the bending is described by the Euler equation where R is the radius ofcurvature, p is density, v is velocity (the subscript j refers to the jet, e to the external medium, g to the galaxy) and h is the scale height over which the ram pressure is transmitted to the jets. Thus, from the jet bending, important constraints on both the jet dynamics and the ICM can be placed. In some cases there is evidence that the radio jets travel first through the galactic atmosphere and then are sharply bent at the transition between the galactic atmosphere and the ICM (Venturi et al. 1989). Bends can occur very close to the nucleus, as in NGC 4869 in the Coma cluster (Feretti et al. 1990), indicating that the bulk of interstellar medium has been stripped by the galaxy during its motion.

The properties of nearby, rich clusters of galaxies containing NAT radio sources has been recently analyzed by Bliton et al. (1998), who derived that NATs are preferentially found in clusters with X-ray substructure. Additionally, NAT galaxies tend to have, on average, velocities similar to those of typical cluster members, instead of high peculiar motions expected if NATs were bent from ram pressure. Thus, they suggested a new model for the NAT formation, in which NATs are associated with dynamically complex clusters with possible recent or ongoing cluster-subcluster mergers. The U-shaped morphology is then suggested to be produced, at least in part, by the merger-induced bulk motion of the ICM bending the jets.

In an effort to understand the orbits of NAT galaxies, O'Dea et al. (1987) examined the directions of NATs with respect to the Abell cluster centres. Assuming that the NATs are indicators of the direction of motion ofthe host galaxies, the resulting random orientation led them to suggest that the overall distribution of NATs is consistent with isotropic galaxy orbits. However, only considering NATs located within 0.5 Mpc, the galaxies exhibited a trend towards radial orbits. Bliton et al. (1998) performed the same analysis and found that the direction of NAT tails are consistent with random orientations in clusters. We note, however, that some clusters show tailed radio galaxies with the tails oriented in the same direction (e.g., A119, Feretti et al. 1999, Fig. 6.5 and Fig. 6.6). This would support the interpretation that bulk gas motion is the

Figure 6.5. Radio image obtained with the VLA at 1.4 GHz of the two NAT radio sources in A119. The arrows indicate the location of the nucleus and of the host galaxy. The two tails show similar orientation.

Figure 6.6. Overlay of the radio and X-ray emission of the cluster A119. The contours represent the two NAT radio galaxies, whereas the grey-scale refers to the X-ray emission from the ROSAT PSPC. The X-ray cluster structure is indication of a recent merger, which could have influence on the formation of the two tailed radio sources.

Figure 6.6. Overlay of the radio and X-ray emission of the cluster A119. The contours represent the two NAT radio galaxies, whereas the grey-scale refers to the X-ray emission from the ROSAT PSPC. The X-ray cluster structure is indication of a recent merger, which could have influence on the formation of the two tailed radio sources.

dominant effect in the formation of the tailed morphology at least in some clusters.

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