2.2 Dynamics of the Local Group

Peebles (1994) estimated _m by calculating the orbits of galaxies in the Local Group based on observed radial velocities, positions, and distances. Shaya et al. (1995) extended this method to a catalog of galaxies within 3000 km/sec. Again this is a method that is conceptually straightforward and independent of H₀ and . Moreover, since the galaxies are nearby, the errors in the distances are relatively small. However, only one (the radial) component of the motion is measured. This method too is based on the assumption that galaxies trace mass. It also assumes that external tidal influences and past mergers are not significant. Furthermore, the question of uniqueness is difficult to address. The estimates based on this method again give low values of _m of ~ 0.15.