Canadian Astronomical Society (CASCA)
Société canadienne d'astronomie (CASCA)
Jo-Anne C. BROWN
University of Calgary
The Magnetic Field in the Outer Galaxy
Observations of synchrotron radiation demonstrate that galaxies have magnetic fields. Our own Galaxy is no exception. The Galactic magnetic field is thought to play a role in both matter and cosmic ray confinement, and hence in overall pressure balance. It is not known how the magnetic field of our Galaxy is generated, nor what its overall structure is. Determining the large scale structure would help identify the origin of the field. Magnetic field reversals (regions of magnetic shear across which the field direction changes by 180 degrees) provide important clues about this large scale structure. Superposed on the large scale magnetic field are smaller scale fluctuations (on the order of 50 parsecs) related to localised motions in the interstellar medium (ISM). In magnetohydrodynamic (MHD) models of the ISM, the assumption is usually made that these small scale fluctuations are isotropic with respect to the large scale field.
The polarisation angle of linearly polarised radiation rotates in a predictable way as it propagates through a magnetized plasma. The process, known as Faraday rotation, is characterized by the measurable quantity of rotation measure (RM), which is determined as the slope of the graph of polarisation angle versus the square of the wavelength. By measuring the RM of polarised compact sources (pulsars, external galaxies, and quasars), we are essentially probing the magnetic field along their lines-of-sight; the more probes there are, the easier it is to reconstruct the intervening field.
As part of my thesis work, I calculated the rotation measure for 380 extragalactic sources in the Canadian Galactic Plane Survey (CGPS; 145 > l > 75, -3.5 < b < 5.5), using the 21 cm polarisation data from the synthesis array at the Dominion Radio Astrophysical Observatory (DRAO). These sources have an average solid angle density of about 1 source per square degree - more than 10 times greater than any previous survey in the Galactic plane. Using these data, the primary goals of my thesis research were to identify reversals in the outer Galaxy (at galactocentric radii greater than that of the Sun), and to test the assumption of isotropy in the small scale field.
In my talk I will review my PhD work, with a specific focus on the questions I addressed, their significance, and the conclusions I was able to reach. I will end with an outline of my more recent work which builds on my PhD studies, by exploring the magnetic field in other parts of the Galaxy.