The Brandeis Radio Astronomy Group conducts research in extragalactic astrophysics and cosmology at wavelengths covering the entire electromagnetic spectrum, but especially at radio wavelengths. We are particularly interested in cores of "active galaxies" or "blazars" - powerful radio galaxies, quasars, BL Lacertae objects, and Seyfert galaxies. Making use of the Very Long Baseline Array (VLBA) of the National Radio Astronomy Observatory, the international Very Long Baseline Interferometry (VLBI) Network, and the VLBI Space Observatory Programme (VSOP), we are studying the angular structure and polarization of extragalactic radio sources in detail at resolutions ranging from arcminutes down to milliarcseconds, corresponding to physical scales from thousands of light-years down to a few light-years.

Our group has pioneered the techniques of making linear polarization images at milliarcsecond resolution using VLBI. Recently, we have extended our techniques to measure the circularly polarized signals that originate from compact radio sources. These experiments are revealing for the first time the magnetic field structure in these compact objects (where apparent motions in excess of the speed of light are often observed), and the nature of the plasma surrounding the massive black hole thought to exist at the center of these sources.

Early programs using the Very Large Array (VLA) in New Mexico include a detailed radio study of the famous gravitationally-lensed double quasar 0957+561. Continuing monitoring of this quasar provides a measurement of the Hubble constant that is completely independent of traditional techniques (Roberts,Lehar, Hewitt, & Burke, Nature, 352, 43 (1991)). The VLA has also been used to make extremely high quality maps of the radio "jets" in many quasars, in order to understand the mechanism by which enormous amounts of energy are transported from the cores to the outer radio lobes.