Thermal stability of cold clouds in galaxy halos

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10^-1.7 M⊙, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the cosmic microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria that are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10^-6 M⊙ necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z ≳ 1 when the temperature of the microwave background was substantially larger than its current value; therefore, the formation and the survival of such clouds to the present epoch remain open questions.