Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies

The H₂ mass of molecular clouds has traditionally been traced by the CO(J = 1-0) rotational transition line. This said, CO is relatively easily photodissociated and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark." We investigate the amount and physical properties of CO-dark gas in two z ∼ 0 disk galaxies and develop predictions for the expected intensities of promising alternative tracers ([C i] 609 μm and [C ii] 158 μm emission). We do this by combining cosmological zoom simulations of disk galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H i and H₂ abundances and CO (J = 1-0), [C i] 609 μm, and [C ii] 158 μm emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field (ISRF) and CR ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e., H₂ gas with W _CO < 0.1 K km s^-1, we find that a significant amount (50%) of the total H₂ mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [C ii], though [C i] also serves as a bright tracer of the dark gas in many instances. At the same time, [C ii] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [C i] suffers the least contamination from diffuse atomic gas and is relatively insensitive to secondary parameters.