4.3 Observed Radio Jet Asymmetries
One of the intriguing characteristics of jets imaged on the sky is that they are often one-sided, almost exclusively so for the high-luminosity AGN (Bridle and Perley 1984), for the parsec scale jets measured with VLBI (Cawthorne 1991), and even at the base of the symmetric jets in low-luminosity objects (Bridle and Perley 1984; Parma et al. 1987). This raises the key question of whether jets are intrinsically one-sided or have one of two intrinsically equal jets brightened by relativistic beaming. If observed superluminal velocities on parsec scales are due to bulk relativistic motion of the emitting plasma, the enormous intensity enhancement from relativistic aberration would inevitably cause jet one-sidedness [Eq. (A8)]. The continuity of sidedness between parsec and kiloparsec scales would then argue for large-scale superluminal motions, which in turn implies kinetic energies many orders of magnitude larger than required by the parsec-scale jets.
The evidence for relativistic speeds on the larger scales is largely circumstantial but not easily dismissed. Wherever one-sided jets are seen on the small scale, they are on the same side as those seen on the large scale, strongly suggesting a connection between the two (Bridle 1992). Studies of the nearby FR I radio galaxy M87 support the hypothesis that its jets are intrinsically two-sided: first, modest superluminal motion has been detected on kiloparsec scales (Biretta et al. 1995) and second, an invisible counter-jet has been inferred from the optically polarized hot spot in the extended radio lobe on the counter-jet side (Stiavelli et al. 1992; Sparks et al. 1992). Similarly, optical evidence for an invisible counter-jet was seen in the superluminal BLRG 3C 120 (Axon et al. 1989).
More general evidence for Doppler enhancement of large-scale jets is found in the depolarization asymmetry in radio lobes (Laing 1988; Garrington and Conway 1991). If jet one-sidedness indicates an oncoming high-velocity jet, then the extended radio lobe on the jet side should be closer to us than the lobe on the opposite side. In 49 of the 69 sources studied by Garrington and Conway (1991), the lobe on the counter-jet side is more depolarized than the one on the jet side, exactly as expected if it were further away and had more depolarizing material along the line of sight. The differential depolarization is explained well by a hot gaseous halo surrounding the radio source (Tribble 1992). It may be possible to ascertain the characteristics of the depolarizing gas independently through sensitive X-ray observations with AXAF, which would confirm that the one-sidedness of most kpc-scale jets is caused by relativistic beaming. We note that in some cases asymmetric radio jet emission correlates spatially with the extended optical emission line gas, which is clearly not beamed, indicating that intrinsic asymmetries are also present at some level in radio sources (McCarthy et al. 1991).
Finally, supporting evidence for the preponderance of relativistic jets
comes from large surveys of radio-loud AGN
(Hough and Readhead
Impey et al. 1991).
Just as one would expect if all jets were relativistic, the ratio of
core radio emission (presumably relativistically beamed) to extended (clearly
unbeamed) radio emission is correlated with optical polarization, optical
power-law fraction, degree/rapidity of variability, jet curvature, and
superluminal motion, and is inversely correlated with linear
7 Lister et al. (1994a) found no correlation between the ratio of core-flux to total-flux and other beaming indicators, but this may be in part because for that particular ratio, core-dominated objects (those with R > 1; Appendix C) are restricted to the limited range 0.5 fcore / ftotal 1.