Unravelling the magneto-ionic fabric of the Milky Way Galaxy

Abstract

Magnetic fields pervade the interstellar medium (ISM) of the Milky Way. These fields interact with various components within the ISM, locking in to the ionized phases of the ISM, forming the magneto-ionic medium. This medium is responsible for a significant input of energy into the ISM. The study of magnetic fields in the ISM of the Milky Way is therefore critical to understanding the energetics and evolution of the Galaxy. This thesis investigates magnetic fields in the Galaxy, and how they interact with a variety of ISM structures. Much remains unknown about the magneto-ionic medium of the Milky Way. This is due, in part, to the difficulty in measuring the strength and structure of magnetic fields. Overall, this limits our understanding of how gas in the Milky Way interacts with magnetic fields, and how those interactions influence Galactic dynamics. Additionally, our location within the Galaxy results in structures on the sky that are large in angular scale and diffuse in nature. The studies presented within this thesis make use of diffuse radio polarimetry made with the Parkes 64m Telescope. Considerable effort has recently been made in the observation of diffuse polarized emission across the entire sky. Polarized radio emission carries a wealth of information on the magneto-ionic medium. Linearly polarized waves experience Faraday rotation as they propagate through the magneto-ionic medium of the Milky Way. Further, the Galaxy itself is a significant source of polarized emission via synchrotron radiation. In combination with measurements of additional ISM tracers, observations of polarization from the Galaxy can unravel the magneto-ionic properties of the Galactic ISM.

I present results from four research papers. I am the primary author of three of these papers. I first describe my technical contribution to the Southern, low- frequency component of the Global Magneto-Ionic Medium Survey (GMIMS-LBS). This spectro-polarimetric survey measures diffuse, linearly polarized emission from 300 to 480 MHz across the entire Southern sky. The second research component of this thesis utilizes the S-band Polarization All Sky Survey (S-PASS), which maps linear polarization at 2.3 GHz. Using these data we map the magnetic structure of the Galactic supershell GSH 006-15+7. In the third research chapter I describe the results of the GMIMS-LBS towards the nearby H ii region Sharpless 2-27 (Sh2-27). In this work we expand on the use of depolarizing regions, such as H ii regions, as powerful distance constraints in the analysis of diffuse polarized emission. Finally, I present analysis of the brightest region in the 408 MHz polarized sky, G150-50. This region exhibits remarkably high fractional polarization, without a corresponding bright region in total intensity. Further, the polarized spectra, as measured by GMIMS-LBS, show remarkable structure. Considering both the morphology and spectral structure of this region, we find that G150-50 is explained by a Faraday caustic. Each of the works presented here highlights both the power and complexity of radio polarimetry. Using diffuse polarized emission we are able to unravel the magneto-ionic medium of the Galaxy. Surveys such as GMIMS provide us with broad-band polarimetry which enables techniques such as Faraday tomography. The features that arise from these observations are often unique to this tracer of the ISM. As we enter the SKA era, understanding how to best utilize and understand these data will be key to solving the mysteries of cosmic magnetism.