Gravitational Microlensing By Dark Clusters In the Galactic Halo

The dark matter in Galactic halos, or some fraction of it, may be in the form of dark clusters which consist of small mass objects. Carr & Lacey (1987) have derived the permissible properties of such systems, and proposed the existence of dark clusters with mass of order ∼ 106M⊙ to explain some of the observed dynamical properties of the stellar disk of the Galaxy. A population of bound systems with mass of ∼ 10-10M⊙ is also an attractive possibility since it is close to the baryon Jeans mass at recombination, which may be the preferred mass scale for the first bound objects to form in the universe. At the present, the existence of dark clusters which consist of brown dwarfs, Jupiters, or black hole remnants of an early generation of stars, is not indicated, nor can be excluded on observational grounds. We describe how dark clusters can be discovered in a sample of gravitational microlensing events in LMC stars. Alternatively, it could provide strict bounds on the fraction of halo mass which resides in such systems. If MACHOs are clustered, the implied degeneracy in their spatial and velocity distributions would result in a strong autocorrelation in the sky position of microlensing events on an angular scale ∼< 20 arcsec, along with a correlation in the event duration. We argue that a small number of events could be enough to indicate the existence of clusters, and demonstrate that a sample of ≃ 10 events would be sufficient to reject the proposal of Carr & Lacey (1987) at the 95% confidence level. If the mass of the hypothesized clusters is much lower than 10M⊙, or the fraction of the dark matter which resides in clusters is small, then a much larger sample of events may be required. Subject headings: The Galaxy dark matter gravitational lensing Magellanic Clouds galaxies: stellar content