| Abstract | We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21star velocity dispersion of σ = 2.2 ± 0.4kms -1. We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61stars, the velocity dispersion is σ = 0.7+0.6 -0.5kms-1. Combining our DEIMOS data with literature values, our final velocity dispersion is σ = 0.4+0.4 -0.3kms-1. We determine a spectroscopic metallicity of [Fe/H] = -1.6 ± 0.1 dex, placing a 1σ upper limit of σ[Fe/H] 0.2dex on any internal metallicity spread. We determine Palomar13's total luminosity to be MV = -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 (Σr η, η = -2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M1/2 = 1.3 +2: 7 -1.3 × 103 M⊙ and a mass-to-light ratio of M/LV = 2.4+5.0 -2.4 M⊙/L⊙. 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 Palomar13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics. © 2011 The American Astronomical Society. All rights reserved. |
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