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]λ158 μm is relatively bright, highlighting a potentially important reservoir of H₂ 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 H₂ mass (M_H2) 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 H₂ determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H₂ that is CO-dark and traced by the [C II]λ158 μm. Results. We demonstrate that while the H₂ traced by CO (1−0) can be negligible, the [C II]λ158 μm can trace the total H₂. We find 70 to 100% of the total H₂ mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by [C II]λ158 μm. The CO-dark gas mass fraction correlates with the observed L_[C II]/L_CO(1−0) ratio. A conversion factor for [C II]λ158 μm to total H₂ and a new CO-to-total-M_H2 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 H₂ 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 efficiency 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 H₂ in the CO-emitting region. [ABSTRACT FROM AUTHOR]