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Description
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 [CII] 158um is relatively bright, highlighting a potentially important reservoir of H_2_ that is not traced by CO (1-0), but residing in the [CII] - emitting regions.Here we aim to explore a method to quantify the total H_2_ mass (MH_2_) in galaxies and learn what parameters control the CO-dark reservoir. We present Cloudy grids of density, radiation field and metallicity in terms of observed quantities, such as [OI], [CI], CO (1-0), [CII] and L_TIR_ and the total MH_2_. We provide recipes based on these models to derive total MH_2_ mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total MH_2_ 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 [CII]158um. We demonstrate that while the H2 traced by CO(1-0) can be negligible, the [CII] 518um 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 [CII] 158um. The CO-dark gas mass fraction correlates with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII] 158um to total H_2_ and a new CO-to-total-MH_2_ as a function of metallicity, is presented. While low metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas, 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 [CII] observations. Accounting for this CO-dark H_2_ gas, we find that the star forming dwarf galaxies now fall on the Schmidt-Kennicuttrelation. Their star-forming efficiency is rather normal, since the reservoir from which they form stars is now more massive when introducing the [CII] measures of the total H_2_, compared to the little amount of H_2_ in the CO-emitting region.
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