Do globular clusters possess dark matter haloes? A case study in NGC 2419

Abstract

We use recently published measurements of the kinematics, surface brightness\nand stellar mass-to-light ratio of the globular cluster NGC 2419 to examine the\npossibility that this Galactic halo satellite is embedded in a low-mass dark\nmatter halo. NGC 2419 is a promising target for such a study, since its extreme\nGalactocentric distance and large mass would have greatly facilitated the\nretention of dark matter. A Markov-Chain Monte Carlo approach is used to\ninvestigate composite dynamical models containing a stellar and a dark matter\ncomponent. We find that it is unlikely that a significant amount of dark matter\n(less than approx. 6% of the luminous mass inside the tidal limit of the\ncluster) can be present if the stars follow an anisotropic Michie model and the\ndark matter a double power law model. However, we find that more general\nmodels, derived using a new technique we have developed to compute\nnon-parametric solutions to the spherical Jeans equation, suggest the presence\nof a significant dark matter fraction (approximately twice the stellar mass).\nThus the presence of a dark matter halo around NGC 2419 cannot be fully ruled\nout at present, yet any dark matter within the 10 arcmin visible extent of the\ncluster must be highly concentrated and cannot exceed 1.1x10^6 Solar masses\n(99% confidence), in stark contrast to expectations for a plausible progenitor\nhalo of this structure.\n