Baryonic Dark Halos: A Cold Gas Component?

We consider the possibility that the dark matter in the halos of galaxies maybe in the form of clusters of \macho s within which are embedded cold, densegas clouds. Microlensing experiments have found evidence that the Galactic halocontains up to half of its mass in the form of low-mass \macho s. A number ofobservational and dynamical arguments point to the existence of hithertounobserved cold gas around galaxies. We show that the cold gas can bestabilized by \macho\ clusters. Within the framework of a simple two-componentmodel, we derive constraints on the \macho\ clusters and on the halo cold gascontent. Typical cluster masses are $\sim 10 \msun$, typical \macho\ masses are$\sim 0.01 \msun$, and the gas content could be up to of order 50 \%. Variouspredictions are given for testing the hypothesis that such objects couldconstitute most of the mass in the dark halos of galaxies. If halos are indeedbaryonic and contain significant amounts of cold gas, they are likely to play amuch more active role in galaxy formation and evolution than is commonlysupposed.