Hydrogen Clouds and the MACHO/EROS Events

Abstract

We propose that the recently reported MACHO/EROS events correspond to gravitational amplification by dark clouds rather than compact objects. These clouds must be very dense with $M\sim 0.1 ~M_\odot$ and $R\la 10^{14}~{\rm cm}$. In all likelihood, the clouds will be members of a family of objects with different sizes and masses. We therefore expect events of longer duration than the ones reported by the MACHO and EROS groups but with light curves very different from the ones derived assuming point mass lenses. We suggest that one such event has already been observed in radio measurements of the quasar 1502+106. The abundances of free electrons, metals, complex molecules, and dust grains are constrained to be very small suggesting that the clouds are formed from a primordial mixture of hydrogen and helium. Cosmic rays and background UV radiation ionize a halo around the cloud. Radio waves from distant sources will be scattered by the electrons in this halo, an effect which may have already been observed in quasars such as 1502+106. We argue that dark clouds are a viable alternative to compact objects for baryonic dark matter in the halo.