Spectral Ageing in the Most Powerful Radio Galaxies

Abstract

Radio-loud AGN represent the most energetic and influential large-scale structures in the Universe. Through physical processes such as AGN feedback, they are suggested to play a major role in the evolution of galaxies. The key physical parameter in these studies is the jet power, which requires an accurately determined age for the source. In this thesis I present a detailed investigation of the dynamics and energetics of two powerful radio galaxies found in cluster environments, in order to probe the well known spectral age/dynamical age discrepancy. Through the use of the high-resolution and newly improved radio interferometer, the JVLA, I present the highest fidelity radio maps of 3C320 and 3C444 to date, which I then use to determine their spectral ages. I also present analysis of the deep and sensitive Chandra observation of the cluster shock surrounding 3C320, used to determine its dynamical age. Applying the most widely used models of spectral ageing I find that, although all models do not provide a reasonable fit to the data, the spectral and dynamical ages can be brought into agreement. I find that the low frequency spectral index (injection index) is relatively flat for these sources, agreeing with previously held assumptions although in contrast to recent work on spectral ageing. There is some suggestion to reject the Kardashev-Pacholczyk model as a valid ageing model for powerful radio galaxies, although this is not robust. I suggest that, since the Tribble model provides consistent fits compared to the other models while providing a more physically realistic description of spectral ageing in the lobes of radio galaxies, it may proceed to be the ageing model of choice for future studies. I argue that the combination of broad-bandwidth data at low and high frequencies with a lobe magnetic field that has a large departure from equipartition estimates can give an agreement between spectral and dynamical ages. I also argue that projection effects and a robust determination of the Mach number of the driven shock by the radio galaxy into its environment are also important factors which may cause a discrepancy in ages. I find no evidence for the conclusions drawn from previous work on powerful radio galaxies in richcluster environments, where it was suggested that their lobes are apparently under-pressured with respect to their environment and therefore require a significant contribution from non-radiating particles for pressure-balance. I therefore suggest that conclusions cannot be made regarding the effect of the environment on lobe particle content of powerful radio galaxies in these rich environments until further samples are studied.