Thermal stability of cold clouds in galaxy halos

Mark Wardle*, Mark Walker

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

33 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)L109–L112
Number of pages4
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 20 Dec 1999
Externally publishedYes


  • Dark matter -
  • Galaxies: halos -
  • ISM: clouds


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