GD-1 spur velocities

Early in the morning I spoke with Ana Bonaca (Harvard) about the amazing velocity data she has taken for stars in the GD-1 stellar stream in the Milky Way halo. As my loyal reader knows, this stream has a spur of stars off the main branch that are consistent with being perturbed away by a massive perturber that flew by. Now she has precise velocity information about stars in the main body of the stream and in the spur. Contrary to our naive predictions, the stream and spur have very similar velocities. But the spur appears to be far lower in velocity dispersion. Is this real? And is this what we expect? We didn't predict it in our theoretical paper on the subject, but then again we didn't look! I can see some arguments that it might be true. Bonaca also sees many other things in the data, like that the GD-1 stream membership is improved dramatically when we have metallicity information.