The transition from atomic to molecular gas with ALMA

The formation of molecular hydrogen in interstellar medium induces the formation of other molecular species, some of which are more easily accessible from the ground than H2. The CO ground state  rotational line integrated intensity is the most common tracer of molecular hydrogen in cloud complexes because this line is easily detected over wide areas. Yet, the combination of the atomic gas directly traced  by the hydrogen hyperfine transition at 21cm and molecular hydrogen traced by CO does not often account for the total gas column, traced by dust emission or by gamma rays. The dark neutral medium (DNM), also called CO-dark gas represents a significant fraction of the total mass of cloud complexes. ALMA enables to probe this gas by detecting absorption lines from molecular species, with  a known abundance relative to molecular hydrogen. The ground state transition of HCO+ at 89.19 GHz has been observed toward 46 background quasars

located in the Taurus, and Chamaeleon molecular complexes. These data confirm the presence of molecular hydrogen even in sight-lines with no detectable CO emission, and show that the DNM is mainly composed of molecular hydrogen. The DNM closely relates to the cold HI phase. Overall,  the total hydrogen column density (HI+2H2) along these sight-lines  is about two-third in atomic form  and one third in molecular form. For sight-lines with CO emission, the CO-H2 conversion factor can be estimated.  We show that 2/3 of the inferred H2 occurred along sightlines with WCO >= 1 K km/s  and recovering 90 % would require detecting CO emission down to 0.2 K-km/s.