Our Milky Way and Andromeda (M31) are the dominant galaxies
in our "Local Group." Both galaxies have many small
"satellite" galaxies, which are believed to orbit about them.
The past history and future fate of the Local Group are not well
known for two reasons. Firstly, the locations and motions
of the galaxies in 3-dimensions are poorly determined.
Secondly, while we can see normal matter from star-light and
thermal emission from interstellar gas and dust, we now
understand that there probably is more unseen "dark matter"
in and around galaxies than normal matter. Since, dark matter
also exerts gravitational forces, without an understanding of the
amount and distribution of dark matter, we cannot understand
how the galaxies have and will move, interact, and even possibly
Using Very Long Baseline Interferometry (VLBI), a technique that
achieves the highest angular resolution in all of astronomy,
RG scientists, in collaboration with radio astronomers in Europe,
have made the most accurate measurements of the motion of
two Local Group galaxies: M33 and IC10. Both of these galaxies
are satellites of the Andromeda Galaxy. Brunthaler et al.
(2005, Science, 307, 1440) have used VLBI observations to
"see" M33 spin and to determine its distance. Also, Loeb et al.
(2005, ApJ, 633, 894) used the measured motion of M33 to
provide constraints on the motion of the Andromeda galaxy and
amount of dark matter in the Local Group.