Recurrent activity in giant radio galaxies

Giant radio galaxies constitute a class of radio sources with linear sizes on the megaparsec scale (Saripalli 1988). In a study of the radio morphologies in giant radio galaxies carried out with the ATCA, Subrahmanyan, Saripalli & Hunstead (1996) drew attention to a variety of morphological features in the giant sources which were indicative of interrupted or episodic nuclear activity and concluded that "giant radio galaxies may have attained their large sizes as a result of a restarting of their central engines in multiple phases of activity along roughly similar directions".

The Westerbork Northern Sky Survey (WENSS) has discovered several cases of "inner-double" or "double-double" giant radio galaxies: the findings suggest that such structures predominantly occur among the larger of radio galaxies. However, only a small number of giant radio galaxies show double-double structures (Schoenmakers et al. 2000). The formation of visible shocks as a restarted beam ploughs through the relic cocoon probably necessitates a high density in the cocoon. This implies that if restarting beams are to be seen in radio images, the initial radio source would have to have evolved over sufficient time so that enough ambient material could have been entrained in its cocoon plasma (Kaiser et al. 2000). This argument is supported by the derivation of consistently larger spectral ages for the giant radio galaxies as compared to smaller radio galaxies (Mack et al. 1998).

Studies of the nuclear recurrence phenomenon in giant radio galaxies are in their early days. As yet, the role of recurrent activity in the formation of giant structures is not clear as also the more fundamental question of whether there is a link between the double-double structure and recurrent central engine activity. We have been following up on our earlier work with detailed ATCA observations of the southern giant radio galaxies whose morphologies indicate the possibility of interrupted activity. An example of such a source is discussed here.

Figure 1 shows an ATCA 20-cm image of B0114-476. The most intense feature is the symmetric, edge-brightened inner double which is seen to stand out as a structure separate from the diffuse lobe emission extending over very much larger scales. We are inclined to believe that the outer diffuse lobes which lack hotspots represent relics of past activity and the inner lobes represent a new injection of energy as a result of the restarting of the nuclear activity following an interruption.

Figure 1: ATCA 20-cm image of B0114-476. The half-power beam size is shown in the lower right corner. Contours at 0.2 mJy/beam x (-1,1,2,3,4,6,8,12,16,24,32,48,96,128,196).

The inner double itself has a linear size of 700 kpc: its size is much larger than typical double radio galaxies. Close to the core of B0114-476, a narrow jet-like feature is seen directed toward the southern inner lobe which also has the brighter hotspots. In the lobes of powerful radio galaxies, one sees a distribution of spectral index with flatter values at the ends and steepening towards the inner regions. However, in the inner lobes of B0114-476, we see no indication of spectral index variation along the length of the lobes: the relative constancy of the spectral index may be symptomatic of the different conditions under which it has formed.

The southern lobe of B0114-476 appears to end along what appears to be a linear bar-like feature perpendicular to the radio axis just south of the core. This bar has a linear extent of almost 1 Mpc and has a steep spectral index below -1.5. The magnetic field lines in the bar are very ordered and oriented along its length; moreover, the percentage polarisation in this feature exceeds 50 per cent. The polarisation properties of the bar may be owing to a compression of the backflowing lobe material; however, the linearity of the feature over 1 Mpc is intriguing. The lobe material appears to have met with an invisible discontinuity in the inter-galactic medium that runs over a distance of a mega-parsec.

One notices that the overall morphologies of the two inner lobes mimic the respective outer lobes: the northern inner and outer lobes are broad in contrast to the southern inner and outer lobes which are cylindrical. This indicates that the external medium on a given side may be affecting the inner and outer lobe morphologies similarly and the differences between the two sides may be attributed to differences in the ambient medium. Whether the hypothesised linear structure in the intergalactic medium to the south of the core causes the lobe asymmetries is unclear.

References

C. R. Kaiser, A. P. Schoenmakers, H. J. A. Rottgering, 2000, Mon. Not. Roy. Astron. Soc. 315, 381

K.-H. Mack, U. Klein, C.P. O'Dea, A.G. Willis, L. Saripalli, 1998 A&A, 329, 431

L. Saripalli, 1988 Ph.D Thesis, Indian Institute of Science, Bangalore, India.

A. P. Schoenmakers, A. G. de Bruyn, H. J. A. Rottgering, H. van der Lan & C. R. Kaiser, 2000, Mon. Not. Roy. Astron. Soc. 315, 371

R. Subrahmanyan, L. Saripalli, & R. W. Hunstead, 1996 MNRAS, 279, 257

Lakshmi Saripalli, Ravi Subrahmanyan (ATNF) and N. Udayshankar (Raman Research Institute, Bangalore)
(Lakshmi.Saripalli@atnf.csiro.au)