Rottmann, Helge: Jet-Reorientation in X-shaped Radio Galaxies. - Bonn, 2002. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Helge Rottmann}},
title = {Jet-Reorientation in X-shaped Radio Galaxies},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2002,
note = {The thesis set out to investigate the nature and genesis of the peculiar class of X-shaped radio galaxies. X-shaped sources are puzzling because of the extremely low number of objects of that type. Three scenarios were investigated:
  1. X-shaped sources are a class by itself and are formed through a mechanisms unlike normal radio galaxies.
  2. X-shaped sources are normal radio galaxies but they are currently in an unusual and/or short lived phase.
  3. X-shaped sources are numerous, but selection effects keep us from identifying more of them.
The backbone of this work is the multi-frequency spectral analysis. Magnetic fields were calculated using the standard minimum energy assumptions. Break frequencies were determined from the synchrotron spectra by fitting spectral models. The break frequencies were then used to calculate spectral ages.
In nearly all sources we have found a gradient of decreasing magnetic fields and spectral indices from the primary towards the secondary lobes. Furthermore, the trend of spectral age clearly indicates increasing ages towards the tips of the secondary lobes. Spectral ages of the secondary lobes have been found to be of the order of a few 10^7 years, which is rather typical for the lobes of 'normal' radio galaxies. Some of the primary lobes (3C223.1, 3C403, NGC326) however are quite young (10^6 years). The young spectral ages of these sources and the short lengths of their primary lobes indicates that possibly the reorientation of their jets is ongoing or has finished only recently. The spectral ages have been used to test the proposed formation scenarios. The required flow speeds in the case of the buoyancy and backflow models is inconsistent with the calculated spectral ages. The predictions of the jet reorientation mechanisms are in excellent agreement with the inferred spectral ages.
In the following we have compared the theoretical distribution of projected source angles for a randomly oriented sample of X-shaped sources with the distribution of observed angles. It has turned out that the studied sample of X-shaped sources must have a distribution of intrinsic angles, which is rather narrow and contains angles between 50 and 90 degrees. This finding does not fit well into any of the proposed formation mechanisms. The absence of observed angles larger than 90 degrees is however predicted by the model of merging binary black holes. The impact of radiation losses on the radiation lifetime of the secondary lobes has been investigated. It has turned out that all sources studied in this thesis have similar secondary lobe fading times of 110-180 Myr. This suggests that their secondary lobes will remain visible throughout the total lifetime of the sources. We have demonstrated that the lobe fading times are considerably shorter (a few 10^7 years) for sources located at higher redshifts or in denser environments. Such fast lobe fading times are consistent with the apparent absence of X-shaped sources at redshifts exceeding z=0.3 and are most probably able to account for the problem of 'missing X-shaped sources'. Finally, we have created images of X-shaped sources that simulate their morphologies during later stages of their evolution. Even though the employed simulation is rather simple, we have obtained simulated maps that are remarkably reminiscent of 'normal' double-lobed radio galaxies. In particular the symmetric lobe extensions that have been reported in a large number of radio galaxies can be reproduced. We speculate that jet reorientation might occur more frequently in radio galaxies than believed up to present.},

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