Deciphering The Last Major Invasion of the Milky Way Galaxy

We present first results from a spectroscopic survey of ∼ 2000 F/G stars 0.5–5kpc from the Galactic plane, obtained with the 2dF facility on the AAT. These data show the mean rotation velocity of the thick disk about the Galactic center a few kpc from the plane is very different than expectation, being ∼ 100km/s, rather than the predicted ∼ 180km/s. We propose that our sample is dominated by stars from a disrupted satellite which merged with the disk of the Milky Way Galaxy some 10-12Gyr ago. We do not find evidence for the many substantial mergers expected in hierarchical clustering theories. We find yet more evidence that the stellar halo retains kinematic substructure, indicative of minor mergers. Subject headings: Galaxy: formation, evolution, stellar content, structure, kinematics and dynamics — stars: kinematics.