Tracking the moving optical photocenters of active galaxies: binary black holes, accretion disks and relativistic jets

We will use the astrometric capabilities of the SIM to answer three key questions about active galactic nuclei: 1) Does the separation of the radio core and optical photocenter of quasars change on the same timescale as their photometric variability, or is the separation stable? 2) Does the most compact optical emission from an active galactic nucleus come from an accretion disk or from a relativistic jet? 3)Do the cores of galaxies harbor binary supermassive black holes remaining from galaxy mergers? We will compare the radio and optical positions of quasars used in the tie between optical and radio celestial reference frames. During the first year after launch, we will be able to show whether the frame tie will be limited by 'astrophysical noise'.