Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
  While the CO(1-0) transition is often used to deduce the total molecular
hydrogen in galaxies, it is challenging to detect in low metallicity galaxies,
in spite of the star formation taking place. In contrast, the [CII] 158 micron
line is relatively bright, highlighting a potentially important reservoir of H2
that is not traced by CO(1-0), but residing in the C+ - emitting regions. We
explore a method to quantify the total H2 mass (MH2) in galaxies and learn what
parameters control the CO-dark gas reservoir. We present Cloudy grids of
density, radiation field and metallicity in terms of observed quantities, such
as [OI], [CI], CO(1-0), [CII], total infrared luminosity and the total MH2 and
provide recipes based on these models to derive total MH2 mass estimates from
observations. The models are applied to the Herschel Dwarf Galaxy Survey,
extracting the total MH2 for each galaxy which is compared to the H2 determined
from the observed CO(1-0) line. While the H2 traced by CO(1-0) can be
negligible, the [CII] 158 micron line can trace the total H2. 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] 158 micron line. The CO-dark gas mass fraction correlates
with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII]
luminosity to total H2 and a new CO-to-total-MH2 conversion factor, as a
function of metallicity, is presented. A recipe is provided to quantify the
total mass of H2 in galaxies, taking into account the CO and [CII]
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, since the reservoir from which they form stars is
now more massive when introducing the [CII] measures of the total H2, compared
to the little amount of H2 in the CO-emitting region.
