Journal Article

The velocity dispersion and mass-to-light ratio of the remote halo globular cluster NGC 2419

H. Baumgardt, P. Côté, M. Hilker, M. Rejkuba, S. Mieske, S. G. Djorgovski and Peter Stetson

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 4, pages 2051-2060
Published in print July 2009 | ISSN: 0035-8711
Published online July 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14932.x
The velocity dispersion and mass-to-light ratio of the remote halo globular cluster NGC 2419

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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 (Tr 0≈ 1010 yr) and large Galactocentric distance (RGC≈ 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 〈vr〉=−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) × 105 M and a global mass-to-light ratio of M/LV= 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 ∼107 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.

Keywords: stellar dynamics; methods: N-body simulations; galaxies: star clusters

Journal Article.  8061 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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