2006ApJ...640L.131C

It has been suggested that cloudlets of cold (≤10 K) H₂and dense (≥10⁷/cm3) molecular gas constitute the dark halos of galaxies. Such gas is extremely difficult to detect because the classical tracers of molecular gas, CO and/or dust grains, have very low abundances and because their emission is exceedingly weak. For this reason, the cloudlet hypothesis remains substantially unproven so far. In this Letter, we propose a new method to probe the presence of cold H₂clouds in galactic halos: the ground transition of ortho-H₂D⁺ at 372 GHz. We discuss why the H₂D⁺ is abundant under the physical conditions appropriate for the cloudlets, and we present a chemical model that predicts the H₂D⁺ abundance as a function of four key parameters: gas density and metallicity, the cosmic-ray ionization rate, and dust grain size. We conclude that current ground-based instruments might detect the ortho-H₂D⁺ line emitted by the cloudlets halo and therefore prove the existence of large quantities of dark baryonic matter around galaxies.