H 2 D + : A Light on Baryonic Dark Matter?

Abstract

It has been suggested that cloudlets of cold (≤10 K) H2 and dense (≥107 cm-3) 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 H2 clouds 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.