Low-mass stars and star clusters in the dark Galactic halo

Abstract

In a previous study it was proposed that the Galactic dark matter being detected by gravitational microlensing experiments such as MACHO may reside in a population of dim halo globular clusters comprising mostly or entirely low-mass stars just above the hydrogen-burning limit. It was shown that, for the case of a standard isothermal halo, the scenario is consistent not only with MACHO observations but also with cluster dynamical constraints and number-count limits imposed by 20 Hubble Space Telescope (HST) fields. The present work extends the original study by considering the dependency of the results on halo model, and by increasing the sample of HST fields to 51 (including the Hubble Deep Field and Groth Strip fields). The model dependency of the results is tested using the same reference power-law halo models employed by the MACHO team. For the unclustered scenario HST counts imply a model-dependent halo fraction of at most 0.5-1.1% (95% confidence), well below the inferred MACHO fraction. For the cluster scenario all the halo models permit a range of cluster masses and radii to satisfy HST, MACHO and dynamical constraints. Whilst the strong HST limits on the unclustered scenario imply that at least 95% of halo stars must reside in clusters at present, this limit is weakened if the stars which have escaped from clusters retain a degree of clumpiness in their distribution.