X-ray Gas Belts and Radio Source Feedback in Low-Redshift Radio Galaxies

  • Ryan Duffy

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


This thesis studies how typical nearby radio sources are participating in radio plasma-gas interactions, with a specific emphasis on understanding the occurrence, morphologies and origins of X-ray gas belts, and their impact upon AGN feedback and fuelling. These excesses (belts) are not likely to be normal undistorted group or galaxy atmospheres.

Firstly, I report measurements from an XMM-Newton observation of the low-excitation radio galaxy 3C 386. The study focusses on an X-ray-emitting gas belt, which lies between and orthogonal to the radio lobes of 3C 386 and has a mean temperature of 0.94 +/- 0.05 keV, cooler than the extended group atmosphere. The gas in the belt shows temperature structure with material closer to the surrounding medium being hotter than gas closer to the host galaxy. I suggest that this gas belt involves a `buoyancy-driven inflow' of part of the group-gas atmosphere where the buoyant rise of the radio lobes through the ambient medium has directed an inflow towards the relic cold core of the group.

This analysis is then expanded to the full low-redshift 3CRR sample which contains 35 radio galaxies. X-ray gas belts are found to be common among the sample, with 15 sources exhibiting an excess of X-ray emitting counts between the lobes but away from the active nucleus. I use Chandra data to investigate the origins of these X-ray belts. I show that all the belts in the sample are primarily composed of thermally emitting gas. In all cases where belt temperature structure can be measured, the outer belt gas is hotter than the inner. It is probable that the belts have a variety of histories including inflow, outflow and mergers. I detect inverse-Compton emission in the plumes of five of the nine FRI radio galaxies in the sample of belted sources, allowing a measurement of magnetic field strength. I find that the amount by which magnetic field strength tends to be lower than the minimum-energy value is similar for FRIs and FRIIs, if minimum energy for both is calculated assuming no protons. Since FRIs in clusters are known to need protons for pressure support with the external medium, these results provide the first evidence that magnetic field strengths are largely oblivious to non-radiating particles in radio galaxy plumes and lobes.

Finally, I use new and archival Chandra observations of Cygnus A, totalling ~1.9 Ms, to investigate the distribution and temperature structure of gas lying within the projected extent of the cocoon shock and exhibiting a rib-like structure. I confirm that the X-rays are dominated by thermal emission with an average temperature of around 4 keV, and have discovered an asymmetry in the temperature gradient, with the southwestern part of the gas cooler than the rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent structure of single origin located inside the cocoon, with a mass of roughly 2 x 10^10 solar masses. I conclude that the gas is debris resulting from disintegration of the cool core of the Cygnus A cluster after the passage of the jet during the early stages of the current epoch of activity. The 4 keV gas now lies on the central inside surface of the hotter cocoon rim. The temperature gradient could result from an offset between the centre of the cluster core and the Cygnus A host galaxy at the switch-on of current radio activity.
Date of Award25 Sep 2018
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorDiana M Worrall (Supervisor)

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