Vol. 643
4. Extragalactic astronomy

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

The CO molecule is the tracer of choice for inferring the amount of star-forming molecular hydrogen gas in our Galaxy and other galaxies with similar metallicities. In low-metallicity systems like dwarf galaxies, however, CO can fail to trace the full reservoir of H2 due to the presence of large quantities of molecular gas where CO has been photodissociated by penetrating far ultra-violet radiation. To account for this CO-dark gas, Suzanne Madden and collaborators have systematically studied the use of CII. Using grids of Cloudy models that span a variety of physical conditions and metallicities, these authors provide recipes for deriving total H₂ mass estimates from CII observations. Using these models, they find that 70% to 100% of the total H2 mass in dwarf galaxies is not traced by CO(1-0) but is well traced by the λ 158 μm line of CII. When this CO-dark gas is accounted for, the dwarf galaxies are found to follow the same Schmidt-Kennicutt relation identified in more metal-rich disk galaxies.