2009MNRAS.396.2051B

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 NGC2419. These data are used to probe the cluster's stellar mass function and search for the presence of dark matter in this cluster. NGC2419 is one of the best Galactic globular clusters for such a study due to its long relaxation time (T_r0~ 10¹⁰yr) and large Galactocentric distance (R_GC~ 90kpc) - 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±0.7km/s and an internal velocity dispersion of σ = 4.14±0.48km/s, leading to a total mass of (9.0±2.2)x10⁵M_☉ 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 NGC2419. 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 ∼10⁷M_☉ inside the central 500pc, 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.