Bradford, J. D.. and Geha, M. and Muñoz, R. R. and Santana, F. A. and Simon, J. D. and Côté, P. and Stetson, P. B. and Kirby, E. and Djorgovski, S. G. (2011) Structure and Dynamics of the Globular Cluster Palomar 13. Astrophysical Journal, 743 (2). Art. No. 167. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120202-110752104
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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 σ = 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 σ = 0.7^(+0.6)_(–0.5) km s^(–1). Combining our DEIMOS data with literature values, our final velocity dispersion is σ = 0.4^(+0.4)_( –0.3) km s^(–1). We determine a spectroscopic metallicity of [Fe/H] = –1.6 ± 0.1 dex, placing a 1σ upper limit of σ_([Fe/H]) ~ 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 (Σ α r^η, η = –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^(+2:7)_(–1.3) × 10^3 M_☉ and a mass-to-light ratio of M/L_V = 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 Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics.
|Additional Information:||© 2011 The American Astronomical Society. Received 2011 August 19; accepted 2011 October 7; published 2011 December 2. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. J.D.B. acknowledges support from the CT Space Grant. M.G. acknowledges support from NSF grant AST-0908752 and the Alfred P. Sloan Foundation. R.R.M. acknowledges support from the GEMINI-CONICYT Fund, allocated to the project N32080010 and from CONICYT through projects FONDAP N15010003 and BASAL PFB-06. Support for this work was provided by NASA through Hubble Fellowship grant 51256.01 awarded to E.N.K. by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. S.G.D. acknowledges a partial support from the NSF grant AST-0909182. We thank Andreas Küpper and Luis Vargas for useful conversation.|
|Subject Keywords:||dark matter; galaxies: kinematics and dynamics; globular clusters: individual (Palomar 13)|
|Official Citation:||Structure and Dynamics of the Globular Cluster Palomar 13 J. D. Bradford et al. 2011 ApJ 743 167|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||02 Feb 2012 19:37|
|Last Modified:||26 Dec 2012 14:46|
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