Baumgardt, H. and Côté, P. and Hilker, M. and Rejkuba, M. and Mieske, S. and Djorgovski, S. G. and Stetson, Peter (2009) The velocity dispersion and mass-to-light ratio of the remote halo globular cluster NGC 2419. Monthly Notices of the Royal Astronomical Society, 396 (4). pp. 2051-2060. ISSN 0035-8711 http://resolver.caltech.edu/CaltechAUTHORS:20090729-115119043
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Precise radial velocity measurements from high-resolution echelle spectrometer on the Keck I telescope are presented for 40 stars in the outer halo globular cluster NGC 2419. These data are used to probe the cluster's stellar mass function and search for the presence of dark matter in this cluster. NGC 2419 is one of the best Galactic globular clusters for such a study due to its long relaxation time (T_r0 ≈ 10^10 yr) and large Galactocentric distance (R_GC ≈ 90 kpc) – properties that make significant evolutionary changes in the low-mass end of the cluster mass function unlikely. We find a mean cluster velocity of〈v_r〉= −20.3 ± 0.7 km s^−1 and an internal velocity dispersion of σ = 4.14 ± 0.48 km s^−1, leading to a total mass of (9.0 ± 2.2) × 10^5 M⊙ and a global mass-to-light ratio of M/L_V = 2.05 ± 0.50 in solar units. This mass-to-light ratio is in good agreement with what one would expect for a pure stellar system following a standard mass function at the metallicity of NGC 2419. In addition, the mass-to-light ratio does not appear to rise towards the outer parts of the cluster. Our measurements therefore rule out the presence of a dark matter halo with mass larger than ~10^7 M⊙ inside the central 500 pc, which is lower than what is found for the central dark matter densities of dSph galaxies. We also discuss the relevance of our measurements for alternative gravitational theories such as Modified Newtonian Dynamics, and for possible formation scenarios of ultracompact dwarf galaxies.
|Additional Information:||© 2009 The Authors. Journal compilation © 2009 Royal Astronomical Society. Accepted 2009 April 15; received 2009 March 24; in original form 2009 January 26. Published Online: 9 June 2009. We thank Tad Pryor and Ed Olszewski for providing their finding charts for NGC 2419. We also thank Iskren Georgiev and an anonymous referee for comments which improved the presentation of the paper. This study was based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. We are grateful to the W. M. Keck Foundation for their vision and generosity. SGD acknowledges a partial support from the NSF grant AST-0407448, and the Ajax Foundation.|
|Subject Keywords:||stellar dynamics; methods: N-body simulations; galaxies: star clusters|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||12 Aug 2009 22:39|
|Last Modified:||26 Dec 2012 11:07|
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