Halo Dark Clusters of Brown Dwarfs and Molecular Clouds

The discovery of Massive Astrophysical Compact Halo Objects (MACHOs) inmicrolensing experiments makes it compelling to understand their physicalnature, as well as their formation mechanism. Within the present uncertainties,brown dwarfs are a viable candidate for MACHOs, and the present paper dealswith this option. According to a recently proposed scenario, brown dwarfs areclumped along with cold molecular clouds into dark clusters -- in severalrespects similar to globular clusters -- which form in the outer part of thegalactic halo. Here, we analyze the dynamics of these dark clusters and weaddress the possibility that a sizable fraction of MACHOs can be binary browndwarfs. Moreover, we point out that Ly-$\alpha$ absorption systems naturallyfit within the present picture.Comment: Latex file. To appear in Astrophysical Journa