Tracing The Total Molecular Gas In Galaxies: [Cii] And The Co-Dark Gas

Context. Molecular gas is a necessary fuel for star formation. The CO(1-0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in spite of the star formation taking place. In contrast, the [C ii]lambda 158 mu m is relatively bright, highlighting a potentially important reservoir of H-2 that is not traced by CO(1-0) but is residing in the C+-emitting regions.Aims. Here we aim to explore a method to quantify the total H-2 mass (MH2) in galaxies and to decipher what parameters control the CO-dark reservoir. Methods. We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as [O i], [C i], CO(1-0), [C II], L-TIR, and the total M-H2. We provide recipes based on these models to derive total M-H2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total M-H2 for each galaxy, and compare this to the H-2 determined from the observed CO(1 0) line. This allows us to quantify the reservoir of H-2 that is CO-dark and traced by the [C ii]lambda 158 mu m. Results. We demonstrate that while the H-2 traced by CO(1 0) can be negligible, the [C ii]lambda 158 mu m can trace the total H-2. We find 70 to 100% of the total H-2 mass is not traced by CO(1 0) in the dwarf galaxies, but is well-traced by [C ii]lambda 158 mu m. The CO-dark gas mass fraction correlates with the observed L-[C ii]/L-CO(1 0) ratio. A conversion factor for [C ii]lambda 158 mu mto total H-2 and a new CO-to-total-MH2 conversion factor as a function of metallicity are presented. Conclusions. While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H-2 in galaxies, taking into account the CO and [C ii] observations. Accounting for this CO-dark H2 gas, we find that the star-forming dwarf galaxies now fall on the Schmidt-Kennicutt relation. Their star-forming e fficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the [C ii] measures of the total H2 compared to the small amount of H-2 in the CO-emitting region.