Studying magnetic turbulence with radio polarimetry

Waelkens, Andre (2009): Studying magnetic turbulence with radio polarimetry. Dissertation, LMU München: Faculty of Physics

Abstract

Magnetised plasma permeates the Universe. It is present in stars, accretion discs, galaxies and clusters of galaxies. This dissertation investigates the characteristics of diffuse magnetised plasma in the inter-stellar-medium (ISM) and the intra-cluster-medium (ICM). To probe turbulence in the ISM and ICM, we develop an estimator for the magnetic tension-force spectrum. It is based on polarised synchrotron emission data in the Faraday-rotation-free frequency regime. The tension force is the dynamically most important magnetic force in subsonic magnetic turbulence. We consider two magnetic field scenarios: a statistically isotropic field distribution, and a statistically isotropic field upon a small mean field. In the latter case the magnetic power spectrum can also be obtained in addition to the tension-spectrum. The method is exact in the ideal case of a homogeneous cosmic ray electron distribution with a power law energy spectrum with a spectral index p=3, and isotropic magnetic fields. The method is applied to mock observations based on magneto-hydrodynamic simulations as a prelude to an application to real observations. We confirm the robustness of the estimator by comparing its output to the corresponding quantities directly computed from the simulation. Furthermore, to investigate the global Galactic field we developed hammurabi. This software tool takes three-dimensional theoretical models for the ISM components and generates mock observations from them. It focuses on the main tracers of our Galactic magnetised plasma such as synchrotron flux, Faraday rotation, dispersion measure and ultra-high-energy-cosmic-ray delflections. Hammurabi therefore permits us to confront models of the Galactic magnetised plasma with a broad range of real observations. We report on a number of works based on this code. In particular, due to simultaneous constraints of rotation measure, polarised and total synchrotron intensity, as well as theoretical predictions on the magnetic field and cosmic-ray electron distribution, the Galactic electron density scale height was suggested to be about twice as large as previously thought. This result obtained with hammurabi was recently confirmed independently by other authors using pulsar dispersion measure data. Finally, we consider the role of the Galactic kinetic Sunyaev Zeldovich (kSZ) effect as a CMB foreground. We analyse the detectability of the Galactic kSZ effect by means of an optimally matched filter technique applied to a simulation of an ideal observation. We obtain a S/N ratio of 0.1, and demonstrate thereby that the Galactic kSZ effect can safely be ignored as a CMB foreground. Furthermore, we rule out any significant contamination of the polarised CMB signal by second scattering of galactic kSZ photons.

• Item Type: Theses (Dissertation, LMU Munich)
• Keywords: MHD, ISM, ICM
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 530 Physics
• Faculties: Faculty of Physics
• Language: English
• Date of oral examination: 5. June 2009
• 1. Referee: White, Simon D. M.
• Signature of the printed copy: 0001/UMC 17869