Kinematic study of the disrupting globular cluster Palomar 5 using VLT spectra

Wide-field photometric data from the Sloan Digital Sky Survey have recently revealed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted. Here we investigate the kinematics of this sparse remote star cluster using high-resolution spectra from the Very Large Telescope (VLT). Twenty candidate cluster giants located within 6' of the cluster center have been observed with the UV-Visual Echelle Spectrograph on VLT-UT2. The spectra provide radial velocities with a typical accuracy of 0.15 km s(-1). We find that the sample contains 17 certain cluster members with very coherent kinematics, two unrelated field dwarfs, and one giant with a deviant velocity, which is most likely a cluster binary showing fast orbital motion. From the confirmed members we determine the heliocentric velocity of the cluster as 58.7 +/- 0.2 km s(-1). The total line-of-sight velocity dispersion of the cluster stars is 1.1 +/- 0.2 km s(-1) ( all members) or 0.9 +/- 0.2 km s(-1) ( stars on the red giant branch only). This is the lowest velocity dispersion that has so far been measured for a stellar system classified as a globular cluster. The shape of the velocity distribution suggests that there is a significant contribution from orbital motions of binaries and that the dynamical part of the velocity dispersion is therefore still substantially smaller than the total dispersion. Comparing the observations with the results of Monte Carlo simulations of binaries we find that the frequency of binaries in Pal 5 is most likely between 24% and 63% and that the dynamical line-of-sight velocity dispersion of the cluster must be smaller than 0.7 km s(-1) (90% confidence upper limit). The most probable values of the dynamical dispersion lie in the range 0.12 less than or equal to sigma/km s(-1) less than or equal to 0.41 (68% confidence). Pal 5 thus turns out to be a dynamically very cold system. Our results are compatible with an equilibrium system. We find that the luminosity of the cluster implies a total mass of only 4.5 to 6.0 x 10(3) M-.. We further show that a dynamical line-of-sight velocity dispersion of 0.32 to 0.37 km s(-1) admits a King model that fits the observed surface density pro le of Pal 5 ( with W-0 = 2.9 and r(t) = 16.'1) and its mass.