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 collide.

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.

Lincoln Greenhill, Avi Loeb, Mark Reid

Heino Falcke, Christian Henkel, Andreas Brunthaler