Astron. Astrophys. 326, 45-50 (1997)

1. Introduction

Although the two radio lobes of powerful double radio sources are broadly similar in many ways, a number of significant and striking differences between them have been revealed by high quality radio/optical observations. Some of the well known types of asymmetries involve observed properties such as the jet's brightness, optical line-emission from the lobes, radio spectral index and spectral curvature, as well as the radio depolarization of the lobes (as summarized recently, e.g., by Komberg 1994 and by Gopal-Krishna & Wiita 1996). The jet asymmetry, which is now generally regarded as being primarily an illusion arising from the jet's bulk relativistic motion and the consequent Doppler boosting of its radiation, provides a useful framework for gaining insight into the origin of the various other types of asymmetry (e.g., Scheuer 1995; Bridle et al. 1994; Saikia 1981).

Laing (1988) and Garrington et al. (1988) first reported a striking correlation which involves the depolarization asymmetry of the radio lobe pair (see, also, Garrington et al. 1991). For a sample of double-lobed radio sources showing one-sided jet (i.e., mostly quasars), they demostrated a very strong tendency for the lobe on the jet's side to depolarize less rapidly with increasing wavelength, as compared to the opposite lobe. This correlation, commonly known as the Laing-Garrington (L-G) effect, is now regarded as a fairly general property of quasars and constitutes a key evidence for the relativistic motion of the jets persisting out to kiloparsec-scale (e.g., Scheuer 1987).

According to the currently popular explanation for the L-G effect, the host galaxies of radio-loud quasars (as well as distant radio galaxies) are supposed to be embedded within diameter hot gaseous haloes, similar to the core of the intra-cluster medium (ICM) of nearby clusters of galaxies, such that the haloes are similar in size to the associated double radio sources. The observed rapid depolarization of the radio lobe associated with the counter-jet is then attributed to the fact that the lobe is seen through a substantially longer depth of magneto-ionic medium, as compared to the lobe on the jet's side which lies on our side and hence, largely in the foreground of the ambient ICM core (cf. Laing 1988; Garrington et al. 1988; Garrington & Conway 1991; Tribble 1992; also, Slysh 1966). Note that this explanation predicts a weaker depolarization asymmetry for radio galaxies since, according to the currently popular unified scheme, they are believed to be the parent population of quasars, so that their radio axes are oriented at large angles from the line-of-sight (Barthel 1989; Antonucci 1989; Urry & Padovani 1995; Gopal-Krishna 1995, 1996). Consistent with this picture, it has been argued that any depolarization asymmetry in radio galaxies is mainly due to a combination of an asymmetric extension of the two lobes from the nucleus and the radially declining density of the hot gaseous coronae associated with the parent galaxies themselves (e.g., Garrington & Conway 1991; Tribble 1992; Strom & Jägers 1988).

1.1. Potential caveats with the canonical explanation

Although, the above explanation for the L-G effect in quasars, invoking an ICM-core like ambient medium, has gained wide popularity, a few potential difficulties seem to question its robustness. In particular, the viability of the model rests on the tacit assumption that in every case, the surrounding ICM core is somehow able to maintain a diameter pretty close to the steadily growing size of the radio source. Even a factor of two difference between the two sizes would erode the L-G correlation seriously (cf. Garrington & Conway 1991). No regulatory mechanism has been proposed, however, that would ensure the needed tight coupling between the two sizes. Secondly, the L-G effect is found to become stronger at higher redshifts (Garrington & Conway 1991) and, while deep optical imaging observations do reveal a clear tendency for radio galaxies and quasars at higher redshifts to occur in cluster environments (e.g., Hill & Lilly 1991; Yates et al. 1989; Yee & Green 1987), the hot intra-cluster medium (ICM) itself appears to actually thin out towards higher redshifts (Castender et al. 1995). Due to this, the basic assumption of the canonical model, namely, the existence of a dense ICM core () around quasars (cf. Garrington & Conway 1991), is fraught with some uncertainty (though, clearly, at least a low-density gas envelope must exist even around distant radio sources). In any event, the origin of the presumed microgauss magnetic field in the ICM, which must, moreover, be ordered on the scale of a few kiloparsecs already by , remains to be explained in a convincing manner.

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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