Astron. Astrophys. 321, 64-70 (1997)

4. Radio observations

The WAT identified with the central D-galaxy was observed at the Molonglo Observatory Synthesis Telescope (MOST) at 843 MHz with a resolution of in 1988, at the VLA at 1.4 GHz in the B/C configuration (resolution of ) and at 4.9 GHz in the C/D configuration (resolution of ) in 1989. The central parts of the source was also imaged at the Australia Telescope at a high resolution () with a 6km array in 1992. In this paper, we will concentrate on the VLA images.

4.1. Analysis of radio data

The 1.4 GHz image was calibrated using 3C48 as the primary calibrator, but a different primary calibrator 3C286 was used for the 4.9 GHz observation. The images were processed using the standard AIPS package for radio synthesis data. Fig. 4 shows the 1.4 GHz image superimposed on the X-ray image. The two tails are reasonably symmetrically located around the centre of the cluster and are bending southwards. The 4.9 GHz image smoothed to the same resolution as the 1.4 GHz image is shown in Fig. 5. The eastern tail disappears below the sensitivity limit in this image, due to the steepness of the spectral index in the tail. The spectral index steepens dramatically from the flat core () to the lobes where . The linear size of the WAT was estimated from the 1.4 GHz image to be kpc. The total radio luminosity of the WAT at 1.4 GHz is estimated to be W Hz^-1, which is just below the FR break. Such moderate radio powers are typical of radio sources associated with cluster dominant galaxies (Heckman 1980) and of medium sized WATs (Burns 1986).

Fig. 4. Radio VLA image of the WAT at 1.4 GHz superimposed on the Rosat PSPC X-ray image in the hard band. The field size is with east to the left and north upwards. The radio images is in contours of (0.8, 3.0, 7.0, 50.0) mJy beam-1. The radio image was smoothed by a Gaussian to a resolution of FWHM (c.f. Fig. 5).

Fig. 5. A VLA contour image of the WAT at 4.9 GHz superimposed on a grey scale hard band PSPC image. The field size is with east to the left and north upwards. The radio contours are (0.07, 0.3, 0.8, 1.0, 2.0) mJy beam-1. The radio image was smoothed by a Gaussian to a resolution of FWHM (c.f. Fig. 4).

A comparison of the redshifts of the D-galaxy and the cluster shows that the D-galaxy is moving away from us at a speed of km s^-1. In general the galaxies with which the WATs are associated move through the intracluster medium (ICM) very slowly, if at all, rarely obtaining velocities greater than about a half their stellar dispersion (which corresponds to 200 km s^-1 ) with respect to the cluster centre (Burns 1982,Eilek 1984,Bothun 1990). WATs were thought to be bent by the ICM through their slow motion in the medium. However, recent detailed studies by O'Donoghue et al. (1993) have shown that this standard model breaks down when examined in detail using the current theories of the physics of the jets. Alternative models suggest that tails may be bent by the bulk flow of velocities km^-1 s in the ICM due to subcluster mergings (Burns et al. 1986; O'Donoghue et al. 1993; Roettiger et al. 1993).

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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