Astrophys. J. 694, 130–143 (2009)

Galactic archaeologists have identified a component of the Milky Way's halo that had been predicted but not seen before. The team, led by Heather Morrison at Case Western Reserve University in Cleveland, sifted through stellar velocity data from surveys going back to 1994, and found a group of stars marching to a different beat from the halo's original inhabitants. These stars were probably part of the outer halo and seem to have arrived at their positions more recently.

Some astronomers had theorized that the halo of stars centred on the Milky Way should contain two components. One, roughly spherical, would not rotate. The other, observed now for the first time, flattened into a thick, slowly rotating disk after the Galaxy's formation when stars from the outer halo drifted inwards.

This new component contains stars with eccentric orbits not found in the rapidly rotating main disk.