Baryonic dark matter in globular clusters

Abstract

A persistent rumor has it that globular clusters cannot contain dark matter. We show here the opposite. Because stars are densely packed, thermal relaxation obtains. Heavy stars tend to sink towards the cluster cores, whereas light objects populate the outskirts. Just like icebergs, globular clusters could therefore contain large amounts of unseen material in the form of light and faint objects. Their properties, as traced for instance by the red giant stars, would not be much affected. We have characterized the presence of heavy and light species with two-component King models. The amount of low-mass stars turns out to be badly constrained from observations of the luminous component. However, the dominance of low-mass stars in the outer regions makes them detectable by the forthcoming infrared telescopes. Those light objects should also induce quite a few gravitational microlensings on the more distant stars of the background. Observation of such events could shed some light on the dark side of globular clusters, and on the missing mass of the galaxy.