Baryonic dark halos: A cold gas component?

We consider the possibility that some of the dark matter in the halos of galaxies may be in the form of cold, dense gas clouds. A number of observational and dynamical arguments point to the existence of hitherto unobserved cold gas around galaxies. The cold gas may be stabilized if it is embedded within dark miniclusters of collisionless objects or particles, or if the thermal balance of the gas is dominated by gas-grain heating. Typical cloud masses are similar to 1 M., and the gas temperature is less than or similar to 10 K. We discuss the evolution of the halo cold gas content and derive constraints on the miniclusters within the framework of a simple two-component model. Various predictions are given for testing the hypothesis that such clouds could contribute substantially to the mass in the outer halos of galaxies. If halos are indeed partly 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.