2009AJ....137.3378O

This paper presents the first observational study of the kinematics of debris from a disrupting globular cluster. Seventy-four bright member candidates of the tidal stream of the cluster Palomar 5, spread over an arc of 8°.5 on the sky, have been observed using high-resolution spectroscopy on the VLT and 17 of them were identified as members of the stream. Their velocities are very close to the radial velocity of the cluster and show a linear trend with position along the stream. We measure the local gradient of the line-of-sight velocity along the stream to be 1.0±0.4 km/s/deg. The stars in the stream show an overall velocity dispersion of 4.7 km/s, which reduces to only 2.2 km/s when removing two stars. This demonstrates that the tidal debris from Pal 5 forms a kinematically cold structure in the Galactic halo, and hence explains the narrowness of the tails. The velocity gradient along the stream provides an important constraint on the orbit of the cluster and thus also on the potential in the Galactic halo. This leads to the conclusion that, in contrast to what was assumed in earlier studies, the cluster's orbit on the sky is not exactly aligned with its tidal tails. A revised model for the orbit of Pal 5 is thus proposed.