The differences in depolarization of the radiation coming from the jet and the counter jet were, according to Laing and Garrington (1988), mainly due to differential Faraday rotation through the hot gas halo surrounding the associated galaxy. This hypothesis can explain the observed depolarization, because you expect that radiation coming from the counter jet would be more depolarized than radiation from the jet side. The difference in path length through the halo gives a natural explanation for this phenomenon. The fact that they observed mostly quasars also explains why most data points in figure 65 lie far under the reference-line. (where DP(jet side)=DP(counter jet side)). In the unification scheme quasars are orientated along the line of sight and therefore the difference in pathlength through the halo can be extremely large.
For radio galaxies the hypothesis would predict the difference in depolarization to be less extreme than with quasars, because they are observed in the plane of the sky. Hence, the plot for radio galaxies would look a bit different. The data points should lie near the reference-line and most of them just under the line, because there is a slight difference in depolarization between the jet side and the counter jet side. If we look at figure 66 we see that this is not what we expected. Most of the data points lie above the reference-line, and the difference in depolarization is substantial.
Presenting the data in this form can be very tricky, because it is difficult (and sometimes impossible) to tell which side of the source is the jet side and which side the counter jet side. You assume for instance that small jet-like structures in the high resolution maps and highest flux densities are seen at the side that is nearer to the observer, but it is not an objective way of looking at the data. That's why we have used another method which is a consistent and a more objective manner for the presentation and interpretation of the data.
We define a quantity as the ratio of lowest depolarization to the highest
depolarization (
). This means that we are not
making a subjective choice about which side the jet side is. We then make a
histogram of the number of sources that fall into a certain range of
. This
is done both for the quasars (data from Laing and Garrington) and for the radio
galaxies (our data plus the two radio galaxies from Laing and Garrington). What
you then expect is a histogram for the quasars where most of the sources lie on
the left side. For radio galaxies most of the sources would lie on the right
side of the histogram. This is indeed found for the quasars, but not for the
radio galaxies (although we only have just 8 sources).
Even with the small number statistics we have there is a trend visible that radio galaxies are not in the right place of the histogram, but mostly on the left side just like the quasars. Therefore, we argument that the simple model of a halo surrounding the radio source, acting like a depolarization medium, is not good enough to explain our observations. In our opinion there has to be internal depolarization as well at a comparable level.
Figure 66: Depolarization of radio galaxies
Now we are going one step further in our investigation and try to look for differences in both lobes. This is done both for the quasars and the radio galaxies. We make separate histograms for lobe A (smallest amount of depolarization) and lobe B (largest amount of depolarization). Again, it is difficult to derive conclusions from small number statistics, but what we see is a trend that, although the histogram of side A looks different than that of side B both for quasars and radio galaxies, the histograms of A are similar for both sources and also the histograms of B show similarities. The careful statement we can make about it, explaining these histograms, is the following interpretation. It is still possible that radio-loud quasars and powerful radio galaxies are the same kind of objects, observed from a different angle of view. Only a halo as a depolarization medium is not enough to explain our observations. There has to be internal depolarization as well at a comparable level. But by looking at the differences between the two lobes we carefully say that its possible that the internal depolarization is different in both lobes. Whether this is due to orientation effects (because on the counter jet side of the source you look along a larger path through the jet) or environmental differences is hard to say, but because we see the same kind of trend with radio galaxies (where the orientation effect should be less dominant) it is in our opinion a difference in the inetrnal environment that is most likely to be responsible. These environmental differences can be due to inhomogenities in the medium like tangled magnetic fields or clumpy clouds of ionized gas.
Figure 67: Histogram: number of sources versus
Figure 68: Histogram: number of sources versus
Figure 69: Histogram: number of sources versus depolarization
Figure 70: Histogram: number of sources versus depolarization
Figure 71: Histogram: number of sources versus depolarization
Figure 72: Histogram: number of sources versus depolarization