Deciphering Galactic Magnetism from Polarisation Structures in the Northern Sky
Date
2020-09-23
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Abstract
The Milky Way is permeated by magnetic fields ranging from stellar scales up to coherence lengths comparable to the Galactic spiral arms. Determining the Galactic magnetic field (GMF) morphology over a range of spatial scales is necessary in understanding the Galaxy’s formation and evolution, and the role of magnetism in its interactions with other Galactic constituents. While ambient magnetic fields do not radiate, they can affect radiation, the results of which are observable in polarisation via Faraday rotation. The aim of my thesis is to explore three-dimensional GMF properties using new observational techniques. I present my analysis of two complementary radio polarisation datasets: the Synthesis Telescope (ST) data from the Canadian Galactic Plane Survey (CGPS) and the Global Magneto-Ionic Medium Survey (GMIMS) single-antenna data. While the CGPS ST data provide high angular resolution, the broad frequency coverage of GMIMS allows for detection of Faraday complexity along the lines of sight. I found that over large regions in the Galactic disk, the extragalactic (EG) and extended emission (XE) rotation measures (RMs) of the ST trace similar patterns with Galactic longitude, indicative of the two observables probing similar volumes through the disk magnetic field. Modelling reveals basic GMF configurations describing the observed ratio of approximately two between the EG and XE RMs. Examining latitudinal variation in both the EG and XE RMs, I found further evidence of the diagonal orientation of the large-scale magnetic field reversal in the inner Galaxy, in agreement with an earlier study of mine. In GMIMS, I found overall large-scale agreement with previous observations of EG RMs at high latitudes. At low latitudes, differences between the GMIMS and the ST RMs are likely the result of beam depolarisation. I observed Faraday complexity along the line of sight towards a depolarising HII (ionised hydrogen) region, which I show to be useful in probing the surrounding GMF. My experiments in spatial filtering of a combined GMIMS and ST dataset revealed that the large-scale field reversal is obscured in GMIMS by an apparent foreground structure of significant angular size.
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Keywords
Galactic magnetism, Interstellar medium, Radio astronomy, Polarisation, Faraday Rotation
Citation
Ordog, A. (2020). Deciphering Galactic Magnetism from Polarisation Structures in the Northern Sky (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.