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 _ { r 0 } \approx 10 ^ { 10 } yr ) and large Galactocentric distance ( R _ { GC } \approx 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 \pm 0.7 km/sec and an internal velocity dispersion of \sigma = 4.14 \pm 0.48 km/sec , leading to a total mass of ( 9.0 \pm 2.2 ) \cdot 10 ^ { 5 } M _ { \odot } and a global mass-to-light ratio of M / L _ { V } = 2.05 \pm 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 \sim 10 ^ { 7 } M _ { \odot } 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 .