Halo dark clusters of brown dwarfs and molecular clouds

The discovery of massive astrophysical compact halo objects (MACHOs) in microlensing experiments makes it compelling to understand their physical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and the present paper deals with this option. According to a recently proposed scenario, brown dwarfs are clumped with cold molecular clouds into dark clusters--in several respects similar to globular clusters--that form in the outer part of the Galactic halo. Here we analyze the dynamics of these dark clusters and address the possibility that a sizable fraction of MACHOs are binary brown dwarfs. We also point out that Ly alpha absorption systems fit naturally within the present picture.