Polarization of the Outer Galaxy

One of the products of the Canadian Galactic Plane Survey (CGPS) is a set of images of the outer Galaxy in Stokes Q and U at four frequencies clustered around 1420 MHz. These can be combined to form rotation measure maps, probing the magnetic field and free electron density of the warm ionized medium. Uyaniker, Landecker, Gray, and Kothes (2003) showed that because of spatially varying rotation measure the polarized emission of extended structures seen in the CGPS originates from within a 'polarization horizon' roughly 2 kpc in radius in the direction of the local spiral arm. This article discusses progress on an extension of the above work to a wider range in both longitude (l in [75 °, 145 °]) and latitude (b in [-3°, 19°]), to investigate the structure of the outer Galaxy's magnetoionic medium. Since extragalactic sources are generally unresolved and thus unaffected by the polarization horizon, comparing their rotation measures to that of diffuse emission in the same region in principle allows us to separate the Faraday rotation of the nearby Galaxy from that of the far outer Galaxy.