Asymmetries of Radio Galaxies

Abstract

In this work, we have studied the asymmetries of radio galaxies which are known to be related to the jet-asymmetry. They are the depolarization asymmetry and the spectral index asymmetry. <br /> The first part of the thesis is concerned with the polarization asymmetry in GRGs and B2 RGs. After the introductory chapter, we discuss the Ricean bias in the fractional polarization and suggest a hybrid solution in the second chapter. The merit of this new solution is the enhanced reliability of the pixel-to-pixel fractional polarization. Using this hybrid solution, we present DP maps of the five GRGs at the end of the chapter. We conclude that the five GRGs are highly depolarized at low radio frequencies, but the depolarization is not as significant as Burn's law predicts. NGC315 shows an interesting characteristic in that the source is NOT depolarized. <br /> The subject of the third chapter is the rotation measure of the five GRGs. After correction for the Galactic contribution to the RM, the GRGs exhibit a few hundred radians per square meters of the Faraday medium. This must be due to field reversals or to changes of the field strength on a scale length of 50 to a few hundred kilo-parsecs. This result indicates that the Faraday media of the GRGs are well resolved, despite the relatively large beam size used.<br /> In the fourth chapter, we study the depolarization asymmetry of the B2 sources. Using the hybrid solution of the second chapter, we confirm the result of Morganti et al.(1997) who discovered the `Laing-Garrington' effect in the B2 sources. The Faraday Rotation Measure (hereafter RM) of the B2 sources is estimated using a frequency range similar to that of the GRG sample, but with better angular resolution. The measured RM is about an order of magnitude lower than that of the B2 sources. We test the correlation between the depolarization asymmetry and the asymmetry of the RM dispersion. There is a trend that the more Depolarization (hereafter DP)--asymmetric source is the more RM dispersion--asymmetric. Again, the trend is not as strong as the prediction of Burn's law. <br /> The fifth chapter is concerned with the high RM that we found in the GRG sample. We discuss the possible processes producing the high RMs. Since the GRGs with a symmetric morphology, e.g. 3C236, also exhibit a high RM, we favour an `internal' mechanism. Our estimate shows that expansion losses can support the amount of magnetic energy in the Faraday media. However, an external process such as large-scale shock flows may also play a role as we saw in the case of NGC315, where the RM becomes even higher. <br /> The sixth chapter deals with the spectral asymmetry. In FR II type sources, there are conflicting results as to this asymmetry. Liu et al.(1991) and Garrington et al.(1991) find a systematic asymmetry where the jet-side spectra are flatter in a sample of FR II type sources. In contrast to these results, Dennett-thorpe(1999) reported in her thesis that there is no systematic asymmetry in a representative FR II sample. <br /> Our concern was that the spectral asymmetry could occur not only through the spectral ageing effect but also through the difference in the injection spectral index. This idea may be more relevant for FR I sources and GRGs than for FR II sources, since the latter have a highly symmetric morphology, hence similar physical conditions are expected in both double lobes. Katz-ston et al.(1997) reported that there are possibly two injection spectra in the lobes of the FR I-type radio galaxy 3C449, based on an analysis of the colour-colour diagram. <br /> We develop a new analysis, the spectral curvature (SCP) - spectral index (alpha) diagram, in order to easily distinguish different injection spectra. We apply this technique to the GRG sample for which we have multiple frequency data at our disposal. The results show that there exists more than just one injection spectrum in the individual sources. Furthermore, we test this SCP-alpha diagram for a sample of 47 CSS sources whose injection spectra had been thoroughly analyzed by Murgia et al.(1999). This test proves that the SCP-alpha analysis is highly reliable. In the GRG sample we find multiple injection spectra using our new SCP-alpha analysis. <br /> On the whole, the subject of this thesis is rather the study of the analysis of the asymmetries, rather than the asymmetries themselves. The behaviour of the two known systematic (jet-asymmetry related) asymmetries is studied. We might verify these results in another way. Recently, RGs were recognized as a promising tool for the process of cosmic structure formation. The linear polarization property of RGs enables us to probe the magneto-ionic intergalactic or intracluster medium. The theory of cosmic particle acceleration is largely based on the observation of the radio spectra of RGs. It will be interesting to investigate a larger and representative sample of RGs with the improved analysis methods suggested in this work.