The dark neutral medium is (mostly) molecular hydrogen

Vol. 675
6. Interstellar and circumstellar matter

The dark neutral medium is (mostly) molecular hydrogen

The transition regime between the atomic and molecular phases of the interstellar medium is not well traced by either HI or CO. It is often referred to as the dark neutral medium (DNM), and its composition has so far remained poorly constrained. To investigate the DNM composition, Liszt and Gerin have combined ALMA absorption HCO⁺(1--0) observations with IRAM 30m CO(1--0) emission data toward about 30 compact extragalactic continuum sources seen with low extinction toward the Galactic anticenter. Using the HCO⁺ absorption intensities as a proxy for the H₂ column densities and prior determinations of the column densities of the DNM, these authors find that the mean DNM and molecular gas column densities are comparable and vary compatibly by factors of 50-100 over the observed sample, while the column density of HI varies by only factors of 2-3. A detailed analysis of the different gas components suggests that although the studied sightlines sample primarily atomic gas with a mean H₂ fraction ≈ 1/3, the DNM is almost entirely molecular.