| Summary: | Some Active Galactic Nuclei (AGN) emit jets of radio-emitting plasma which travel at relativistic speeds. In clusters of galaxies these AGN are surrounded by a thermal gas – the intra-cluster medium (ICM) – which bathes the galaxies. The temperature of the ICM is 10<sup>7</sup> to 10<sup>8</sup> K and so emits X-rays. The AGN jets plough into the ICM and inflate bubbles of relativistic plasma. These bubbles eventually detach and rise up through the cluster. The high spatial resolution of the <i>Chandra </i>X-ray Observatory allows a detailed analysis of the morphology of the ICM and the nature of the interaction. Some bubbles do not have any GHz radio emission as the synchrotron emitting electrons have aged, and these are known as “ghost” bubbles. From thermodynamic arguments, the energy required to create the bubbles can be calculated, as can their ages, and hence the mechanical power of the AGN. The particle content of the plasma within the bubbles was investigated under the assumption that they are in pressure with the surrounding ICM. The energy density of the synchrotron-emitting plasma can be calculated from radio observations and compared to the ICM calculated pressure from the X-ray observations. Limits were obtained on the radio <i>k/f</i> for a sample of clusters, where <i>k</i> is the ratio of the total particle energy to that in synchrotron electrons, and <i>f</i> is the volume filling fraction of the relativistic plasma. The evolution of this ratio was investigated in two clusters which have both young and ghost bubbles. To investigate whether any non-relativistic particles are intrinsic to the jet or entrained from the ICM, a comparison between the radio emission from the core and the jet power, as calculated from the expansion of the radio bubbles is made. In two clusters limits were set on the type of jet present, assuming that the low energy cut-off in the electron energy spectrum, γ<sub>min</sub>, is ~ 1. As previous attempts to determine the matter content of jets have a lower limit of γ<sub>min </sub>≳ 100, the effect of changing γ<sub>min</sub> on the results was investigated.
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