Is the galactic halo baryonic?

Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizeable fraction of the Galactic halo is in the form of massive astrophysical compact halo objects (MACHOs) with mass less than about 0.1 Msun. Here, the authors argue that molecular clouds (mainly of H2) located in the Galactic halo can contribute substantially to its total mass. They outline a scenario in which dark clusters of MACHOs and molecular clouds naturally form in the halo at large galactocentric distances. Possible ways of detecting MACHOs via infrared emission and molecular clouds via the induced γ-ray flux are discussed. Molecular clouds located in the dark halo of M31 could be discovered through cosmic background radiation anisotropies or emission lines in the microwave band.