Baryonic Dark Halos: Machos and Cold Gas?

Abstract

We consider the possibility that the dark matter in the halos of galaxies may be in the form of clusters of \macho s within which are embedded cold, dense gas clouds. Microlensing experiments have found evidence that the Galactic halo contains up to half of its mass in the form of low-mass \macho s. A number of observational and dynamical arguments point to the existence of hitherto unobserved cold gas around galaxies. We show that the cold gas can be stabilized by \macho\ clusters. Within the framework of a simple two-component model, we derive constraints on the \macho\ clusters and on the halo cold gas content. 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 \%. Various predictions are given for testing the hypothesis that such objects could constitute most of the mass in the dark halos of galaxies. If halos are indeed baryonic and contain significant amounts of cold gas, they are likely to play a much more active role in galaxy formation and evolution than is commonly supposed.