Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-02-27
Astrophys.J. 640 (2006) L131-L134
Astronomy and Astrophysics
Astrophysics
Accepted for publication in ApJ Letters
Scientific paper
10.1086/503628
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.
Ceccarelli Cecilia
Dominik Carsten
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