Radio Observations of Jets: Large Scales

4.2. Depolarization Asymmetry in FRII Sources

Figure 4 shows the current evidence for a relation between jet and depolarization sidedness in FRII sources.

Figure 4. A plot of Faraday depth on the counter-jet side against Faraday depth on the jet side for a sample of sources with strong, one-sided jets. Data are taken from Garrington et al. (1991) and references therein. The units are cm^-3 µG pc.

Two lines of evidence suggest that the explanation of depolarization asymmetry is more complicated than this. The first is that, in radio galaxies (most of which do not have detected jets), the lobe which is closer to the nucleus shows stronger depolarization (Pedelty et al. 1989a, b; Liu & Pooley 1991a; see Figure 5). Light-travel effects cannot have anything much to do with the observed depolarization asymmetry, however, since McCarthy et al. (1991) and Liu & Pooley (1991b) showed that the closer, more depolarized lobe is almost always on the side of the source with more extended line emission, again in a sample of radio galaxies. The most plausible hypothesis is that the separation and depolarization asymmetries in these sources results from a difference in external density on opposite sides of the nucleus. The correlation between jet and depolarization sidedness appears to be an unrelated effect, since Figure 6 shows that there is no strong tendency for the shorter side of sources with strong, one-sided jets to depolarize more rapidly than the longer side. Similarly, Garrington & Conway (1991) showed that there is no tendency for the extended emission line gas to correlate with jet sidedness in a sample with strong jets.

Figure 5. A plot of Faraday depth on the shorter side of the source against Faraday depth on the longer side for a sample of FRII sources in which jets are weak or undetected. The data are taken from Liu & Pooley (1991a) and Pedelty et al. 1989a, b.

At present, it seems reasonable to adopt a modified hypothesis:

Jets are symmetrical and relativistic.
The distribution of arm lengths is determined by differences in external density from side to side, and this also causes the emission-line asymmetries.
Sources without prominent observed jets (powerful radio galaxies, for the most part) are close to the plane of the sky and the differential Faraday depths between their lobes are dominated by density differences.
Sources with prominent jets (mostly quasars) are selected to be closer than about 45° to the line of sight. Differential Faraday depths are determined primarily by path length effects.

It is expected that the two effects will act in opposite senses in a few sources, and indeed the few exceptions to the depolarization-jet sidedness relation (e.g. 3C 337 (Pedelty et al. 1989b) and 3C 441 (Garrington et al. 1991)) are radio galaxies with very weak jets, in which the shorter lobe shows more depolarization. The hypothesis would also be consistent with simple "Unified models" in which radio galaxies and quasars are drawn from the same population but the former are closer to the line of sight (Scheuer 1987; Barthel 1989). There should also be a tendency for depolarization to be correlated with distance from the nucleus for the counterjet lobe only in sources with strong jets. The counterjet lobe is seen through the bulk of the galaxy halo and the Faraday depth is larger either if it is seen in projection close to the nucleus or if the external density is larger on that side of the source. The jetted lobe, on the other hand, is in front of most of the halo, and no such correlation should be seen. This effect is indeed observed (Garrington et al. 1991).

Figure 6. A plot of Faraday depth on the shorter side of the source against Faraday depth on the longer side for a sample of FRII sources with strong, one-sided jets (Garrington et al. 1991).

There is also a correlation between spectral index and depolarization, in the sense that the lobe with the steeper spectral index depolarizes more rapidly with increasing wavelength (Liu & Pooley 1991a; Garrington et al. 1991). This holds both for jetted and unjetted sources and may, therefore, pose a problem for the simple hypothesis. It is important to decide whether the correlation applies to the lobe spectra in jetted sources, since the lobe emission is presumed to be isotropic. Present data are inadequate to decide this question, since lobe spectra are contaminated by emission from jets and hot-spots (both of which tend to have flatter spectra). Both of these components are stronger on the jetted side, by definition and observation, respectively (Laing 1989), so to be consistent, we need to postulate that emission from both the jet and the hot-spot are beamed. We therefore need to add two additional ideas:

There is an intrinsic correlation between spectral index, arm length and depolarization sidedness, possibly resulting from density differences. This dominates in unjetted sources.
The correlation between spectral index, jet and depolarization sidedness in jetted sources is due to the increased prominence of jet and associated hot-spot emission. This idea can be tested straightforwardly by higher-resolution observations.