Astron. Astrophys. 336, 455-478 (1998)

1. Introduction

Giant Radio Galaxies (GRGs, e.g. Saripalli et al. 1986, Subrahmanyan et al. 1996) are the largest members of the radio galaxy population, with a (projected) linear size  Mpc ¹. At low redshift () some thirty of these large sources are known. The most extreme case is the radio source 3C 236 at a redshift of , which has a projected linear size of 5.7 Mpc (Willis et al. 1974, Strom & Willis 1980, Barthel et al. 1984).

GRGs are interesting objects to study. First, at low redshift their angular size is several arcminutes or larger, which allows detailed studies of the different components of their radio structure, such as their jets (e.g. NGC 6251, Perley et al. 1984) and their lobe emission (e.g. Mack et al. 1997a) using a variety of radio instruments. Secondly, because of the large physical size of GRGs, they have expanded well out of the denser central regions of the clusters that they reside in, into a much less dense Intergalactic Medium (IGM). This makes GRGs a powerful and unique tool to probe the low-density medium at a large distance from their host galaxies, which is inaccessible to current X-ray instruments. Thirdly, their active galactic nucleus (AGN) must be approaching the endstage of its radio active phase. GRGs can therefore provide important information on the properties of old AGN. Lastly, several of these large sources have been identified with quasars (e.g. 4C 34.47, Jägers et al. 1982) or Broad Line Radio Galaxies (e.g. 0319+412, de Bruyn 1989). This makes them important test cases for the orientation dependent unification schemes for radio loud AGN (e.g. Barthel 1989). These state that quasars and radio galaxies are mostly alike, but that quasars have their radio jets oriented closer to the line of sight than radio galaxies. A natural consequence of this is that quasars are not expected to have large projected linear sizes.

There are now many indications that the ambient medium of the radio lobes of GRGs has a very low density ( cm^-3). Probably the strongest evidence is provided by observations of the Rotation Measure (RM) and the depolarization towards these sources. The measured RMs are small, with values which are usually  rad m^-2 (e.g. Klein et al. 1994, Strom & Willis 1980). Depolarization occurs only at wavelengths above 10-20 cm (Willis & Strom 1978, Strom & Willis 1980, Jägers 1986, Klein et al. 1996). This combination can only be explained satisfactorily by a very low density of thermal electrons along the line of sight.

Further indications for a low density environment result from spectral age analyses of GRGs. Typical spectral ages for GRGs have been found to be  yrs (e.g. Mack et al. 1998), which translates into expansion velocities of . Using the assumption of ram pressure equilibrium at the head of the jet (e.g. Miley 1980), external densities of  cm^-3 are commonly found for GRGs. This method suffers from many assumptions that have to be made, such as equipartition between the energy density of the relativistic particles and the magnetic field, the filling factor of the radio lobes, the fraction of energy in heavy particles, the area of the bow shock, and so on. It is therefore often criticized (e.g. Eilek et al. 1997) and its result should be interpreted with care.

Many clusters have large and bright X-ray haloes, often extending to distances of more than 1 Mpc from their centers and containing cooling flows (e.g. Fabian 1994). X-ray studies of some GRGs (e.g. NGC 6251; Mack et al. 1997b), however, have found only weak thermal X-ray emission around the host galaxies. GRGs are therefore probably not inside rich clusters with dense cores, and the lobes are thus not likely to be found in dense environments. Subrahmanyan et al. (1996) studied a small sample of GRGs on the southern hemisphere. By studying the surface density of optical galaxies in the neighbourhood of the GRG host galaxies using the UKST plates, they conclude that they do not reside in rich clusters.

GRGs have relatively low radio powers (Saripalli et al. 1986; Subrahmanyan et al. 1996), usually around or below the luminosity which divides FRI and FRII type radio galaxies ( W Hz^-1 at 178 MHz; Fanaroff & Riley 1974). Because of their large sizes and relatively low radio power, the surface brightness of GRGs is low. This is why they are so difficult to find in most radio surveys. In recent years, the Westerbork Northern Sky Survey (WENSS, Rengelink et al. 1997) has mapped the sky above  declination at a frequency of 327 MHz. WENSS has a sensitivity of 18 mJy (5) and a beamsize of , with the declination. At low frequency, the bulk of emission of radio galaxies originates in the extended radio lobes. This makes the WENSS ideally suited to find large, low surface brightness radio galaxies such as GRGs. The first discovery of such an object (WNB 1626+5153) has been reported by Röttgering et al. (1996). This initiated a project aimed at finding and studying a large, uniformly selected, sample of giant radio sources from the WENSS.

Here we report on the discovery and subsequent analysis of the radio source WNB 0313+683, which we have identified as an FRII-type radio galaxy with a linear size of 2.0 Mpc. It has several remarkable properties, among which there is a large flux asymmetry of the radio lobes and a prominent, inverted spectrum, radio core. In Sect. 2, we will present the radio and optical data that we have collected on this object. Sect. 3 describes a first analysis of these data, including a new way to measure Rotation Measures. It also presents some of the derived physical properties. In Sect. 4 we derive the advance velocities of the hotspots and the age of the source from a spectral index analysis. Sect. 5 then discusses the observed depolarization towards WNB 0313+683. A discussion on the properties of the radio core is given in Sect. 6. We argue that WNB 0313+683 may currently be in a new phase of radio activity. Finally, a summary and our conclusions are presented in Sect. 7.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
helpdesk.link@springer.de