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

Abstract

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 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 (M H 2) 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 H 2. We provide recipes based on these models to derive total M H 2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total M H 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 [C ii]λ158 µm. Results. We demonstrate that while the H 2 traced by CO (1−0) can be negligible, the [C ii]λ158 µm 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]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 H 2 and a new CO-to-total-M H 2 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 H 2 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 H 2 compared to the small amount of H 2 in the CO-emitting region.

[C ii] and CO Emission along the Bar and Counter-arms of NGC 7479*

“…This idea is supported by the observation that this region has the lowest best-fitting SED metallicity of all the regions observed (see Table 2), a condition that may be conducive to CO-dark molecular gas (Wolfire et al 2010). Recent studies of lowmetallicity regions in galaxies (Madden et al 2020) show that most of the CO is dissociated in these environments, making [C II] a better tracer of molecular gas in these particular regions. Region I, which lies close to the end of the continuum radio jet in the south, will be discussed in the next subsection in the context of possible ISM heating by the jet.…”

“…The region shaded in yellow cannot be explained in terms of pure PDR emission, but requires either an excess of [C II] emission or a deficit of CO emission. Most of the normal and star-forming local galaxies lie in the blue region, while higher ratios are measured for dwarf galaxies and lower metallicity galaxies (Madden et al 2020).…”

“…For example, in addition to correcting for the fraction of ionized gas, this line is sensitive to purely neutral atomic gas (Croxall et al 2017). It can also reveal the presence of molecular gas in regions of low metallicity that are usually CO-dark (Wolfire et al 2010;Jameson et al 2018;Chevance et al 2020;Madden et al 2020) and may form a significant fraction of the diffuse ISM in our own Galaxy (Pineda et al 2013).…”

Extremely weak CO emission in IZw 18

“…We note that [C ii] is considered to be a good tracer of CO-dark molecular gas (e.g. Cormier et al 2015;Accurso et al 2017b;Zanella et al 2018;Madden et al 2020). Even though [C ii] can originate from both ionised gas and neutral gas, simulations and models have found that the ionised fraction does not go beyond 50%, even for metal-poor galaxies (Accurso et al 2017a;Cormier et al 2019).…”

ALMA Lensing Cluster Survey: A spectral stacking analysis of [C II] in lensed z ∼ 6 galaxies

“…As carbon has an ionisation potential of 11.2 eV, below that of hydrogen, [C ii] is detected in regions of both predominantly neutral and ionised gas. The line luminosity has been found to correlate with the star formation rate (SFR) in normal, star-forming galaxies (Stacey et al 2010;De Looze et al 2011Herrera-Camus et al 2015;Schaerer et al 2020) and has also been proposed as a gas mass tracer (e.g., Zanella et al 2018;Madden et al 2020). As the emission wavelength is redshifted into the submillimetre and millimetre bands, the [C ii] is recognised as an important probe of the ISM of high-z galaxies.…”