Broadband Radio Polarimetry as a Probe of Magnetised Plasma Towards Powerful Radio Sources

Abstract

This thesis considers whether “Faraday complexity” — information imprinted as a frequency-dependent signal on linearly polarised radiation by Faraday rotation as it propagates through magnetised cosmic plasmas — can be exploited as an effective probe of magneto-ionised structure in active galactic nuclei (AGNs), radio galaxies, and intervening material. A blind search for Faraday complexity amongst 563 radio sources over 1.3–2 GHz results in a signal-to-noise-limited detection rate of 12%. The Faraday-complex sources are found to preferentially lie behind magnetised, turbulent interfaces between neutral and ionised gas in the Galactic interstellar medium. A targeted survey of 36 objects over 1.3–10 GHz reveals striking Faraday complexity in all of the sample sources. The characteristics of the radio emission, including the spectral index of the radio emission and its linear extent, temporal variation in polarisation, and the Faraday-thickness of emission components, suggests that Faraday complexity arises in the AGNs themselves at these frequencies. The data supply strong constraints on the global magnetised structure of AGN and their jets. Observations of the nearby radio galaxy Fornax A over 1.3–3.1 GHz reveal complex frequency-dependent polarisation structure. Filaments of strong depolarisation are revealed to be real physical structures, likely caused by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities at the lobe / intergalactic medium interface. Faraday complexity also exists away from the depolarised filaments in the body of the lobe, and I argue that this must be caused by thermal plasma in the lobe that is well-separated from the synchrotron-emitting plasma. I conclude that broadband observations of Faraday complexity do indeed provide a singular probe of magnetised plasmas in diverse cosmic environments.