The linearly polarized emission allows us to determine properties of the magnetic field in the emitting plasma, since the observed electric field is orthogonal to the magnetic field in the source. Furthermore, by observing at multiple frequencies simultaneously, we may investigate properties of the gas surrounding the emitting plasma via the Faraday effect. This effect is that if linearly polarized electromagnetic radiation passes through a magnetized plasma, the electric vector will rotate, characterized by a quantity known as the rotation measure, which is a function of the magnetic field strength and the electric charge density. I was one of the first to successfully map the rotation measure of radio sources.
My primary source of interest was the quasar 3C309.1.
What are AGN? In simple terms, these are galaxies whose cores are so luminous that they outshine the rest of the galaxy, thereby appearing starlike in optical images. Quasars are the best known type of AGN. These show strong line emission. Another broad class of AGN is the BL Lacertae class, which show little or no line emission.
The widely accepted model of AGN is that they are powered by supermassive (say, 106 solar masses) black holes. Hot gas falls into this black hole, forming an accretion disk around it. If we assume the black hole to be spinning, then the accretion process will generate jets of plasma along the spin axis. We further assume that these jets are magnetized and that the plasma is moving at highly relativistic speeds. The magnetization causes synchrotron emission from the jets, which we may observe. The high relativistic velocity means this emission is greatly amplified in a cone about the jet axis, resulting in the extreme brightness of the core in an AGN.
What is VLBI? This is interferometry using telescopes separated by continental distances. For example, the most commonly used instrument for VLBI observing is the VLBA, which includes telescopes from the US Virgin Islands to Hawaii.
In interferometry, signals received at a pair of telescopes are mixed, forming an interference pattern. By using as many telescopes as possible and observing over a long period of time, we may measure this interference pattern at a range of points in the so-called Fourier space. This space is analogous to a lens or mirror. From these measurements, we may obtain an image of the emission pattern. Interferometry has the property that increasing the separation between telescopes increases the resolution of the resultant image. Using the VLBA, one obtains images with a resolution of about 2 milliarcseconds when observing at a wavelength of 6 cm. For comparison, a full moon is about 30000 milliarcseconds in diameter.