Deciphering the Last Major Invasion of the Milky Way

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Abstract
We present first results from a spectroscopic survey of ~2000 F/G stars 0.5-5 kpc from the Galactic plane, obtained with the Two Degree Field facility on the Anglo-Australian Telescope. These data show the mean rotation velocity of the thick disk about the Galactic center a few kiloparsecs from the plane is very different than expected, being ~100 km s-1 rather than the predicted ~180 km s-1. We propose that our sample is dominated by stars from a disrupted satellite that merged with the disk of the Milky Way some 10-12 Gyr 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.
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