SIM will obtain proper motions for a sample of 27 galaxies, the first proper motion measurements of galaxies beyond the satellite system of the Milky Way. These measurements lead to knowledge of the full six-dimensional position and velocity vector of each galaxy. In conjunction with new gravitational flow models, the result will be the first total mass measurements of individual galaxies. SIM measurements will strongly constrain galaxy total masses and make a locally determined estimate of the global mass density of the Universe. There will be sufficient information to measure dark matter mass to the outermost edges of galaxies, and to discern the sizes of extended halos that may extend far beyond the stars and gas of the observable galaxies.
Observations of extragalactic objects with SIM will allow the study of their parsec-scale structures in the optical waveband for the first time. Are the optical photo-centers of quasars compact and positionally stable on the micro-arcsecond level? There are many theoretical reasons to expect that the most variable quasars and AGN will show motion at several 10s of microarcsec. Changes in their optical positions will be resolvable by SIM at the few mas level. In the standard theory of extragalactic radio sources, emission from quasars and AGN is assumed to be powered by a central engine (presumably a black hole) where energetic phenomena occur. The origin of the optical wavelength radiation from AGN identified with compact radio sources is not well established. There are several possibilities: 1) the optical radiation is thermal emission from an accretion disk; 2) the optical radiation is non-thermal emission from a magnetized corona or wind emanating from the central region of the accretion disk; or 3) the optical radiation is emission from a relativistic jet beamed toward the observer. SIM observations will allow a direct test of this model.
Since the parallax of a quasar is immeasurably small, SIM does not need to determine it. A quasi-inertial reference frame, formed by stars within a few degrees of the main target, will be adequate. Since AGN variability occurs on a variety of timescales, SIM will be able to deliver meaningful results relatively quickly. As the mission progresses, the results can be refined by tying those reference stars to the astrometric grid. For quasar studies SIM will allow doubly differential astrometry, by searching for color-dependent position shifts (across the optical waveband) and their vector time-dependence on the sky. This experiment is differential in both position and wavelength.
SIM will also search for direct evidence of binary black hole systems. In these systems, one or both of the massive black holes may have an active galactic nucleus (AGN) surrounding it. The two holes, along with their AGN, orbit their common center of mass. Significant proper motions, detectable by SIM, occur only for relatively close binaries. This might occur near the end of a galactic merger event, when the two galactic nuclei themselves merge, with a characteristic timescale of a few hundred million years. Rough estimates, based on the circumstantial evidence currently available, indicate that displacements of 10 mas or more (readily detectable with SIM) may be present in a number of AGN. The best candidate is probably OJ287, with an inferred period of 24 years from variability monitoring, and a mass of 109 solar masses. During 5 years of SIM monitoring, the expected orbital displacement is about 15 mas.
SIM Science Investigations
