STRUCTURE AND DYNAMICS OF THE GLOBULAR CLUSTER PALOMAR 13

We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of sigma = 2.2 +/- 0.4 km s(-1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is sigma = 0.7(-0.5)(+0.6) km s(-1). Combining our DEIMOS data with literature values, our final velocity dispersion is sigma = 0.4(-0.3)(+0.4) km s(-1). We determine a spectroscopic metallicity of [Fe/H]= -1.6 +/- 0.1 dex, placing a 1 sigma upper limit of sigma([Fe/H]) similar to 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M-V = -2.8 +/- 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (Sigma alpha r(eta), eta = -2.8 +/- 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M-1/2 = 1.3(-1.3)(+2:7) x 10(3) M-circle dot and a mass-to-light ratio of M/L-V = 2.4(-2.4)(+5.0) M-circle dot/L circle dot. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics.