H_2D^+: a light on baryonic dark matter?

Abstract

It has been suggested that the dark halos of galaxies are constituted by cloudlets of cold (< 10 K) H_2 and dense (> 10^7$ cm^{-3}) molecular gas. Such gas is extremely difficult to detect, because the classical tracers of molecular gas, CO and/or dust grains, have very low abundances and their emission is exceedingly weak. For this reason, the cloudlet hypothesis remains so far substantially unproven. In this Letter we propose a new method to probe the presence of cold H_2 clouds in galactic halos: the ground transition of ortho-H_2D^+ at 372 GHz. We discuss why the H_2D^+ is abundant under the physical conditions appropriate for the cloudlets, and present a chemical model that predicts the H_2D^+ abundance as function of four key parameters: gas density and metallicity, cosmic ray ionization rate and dust grain size. We conclude that current ground-based instruments might detect the ortho-H_2D^+ line emitted by the cloudlets halo, and prove, therefore, the existence of large quantities of dark baryonic matter around galaxies.

Accepted for publication in ApJ Letters

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Cited by in corpus (4)

• Possible detection of the M31 rotation in WMAP data
• Far-Infrared detection of H2D+ toward Sgr B2
• CMB as a possible new tool to study the dark baryons in galaxies
• A computed line list for the H2D+ molecular ion