The velocity dispersion and mass-to-light ratio of the remote halo
  globular cluster NGC 2419
  Precise radial velocity measurements from HIRES 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/sec and an internal velocity dispersion of \sigma = 4.14 +- 0.48 km/sec,
leading to a total mass of (9.0 +- 2.2) * 10^5 Msun 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 Msun 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 MOND, and for possible formation scenarios of ultra-compact
dwarf galaxies.
