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