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The $\rm H_2$ mass of molecular clouds has traditionally been traced by theCO(J=1-0) rotational transition line. This said, CO is relatively easilyphotodissociated, and can also be destroyed by cosmic rays, thus rendering somefraction of molecular gas to be "CO-dark". We investigate the amount andphysical properties of CO-dark gas in two $z \sim 0$ disc galaxies, and developpredictions for the expected intensities of promising alternative tracers ([CI609 $\mu$m and [CII] 158 $\mu$m emission). We do this by combining cosmologicalzoom simulations of disc galaxies with thermal-radiative-chemical equilibriuminterstellar medium (ISM) calculations to model the predicted H~\textsc{i} and$\rm H_2$ abundances and CO(J=1-0), [CI] 609 $\mu$m and [CII] 158 $\mu$memission properties. Our model treats the ISM as a collection of radiallystratified clouds whose properties are dictated by their volume and columndensities, the gas-phase metallicity, and the interstellar radiation field andcosmic ray ionization rates. Our main results follow. Adopting anobservationally motivated definition of CO-dark gas, i.e. $\rm H_2$ gas with$W_{\rm CO} < 0.1 $ K-km/s, we find that a significant amount ($> 50\%$) of thetotal $\rm H_2$ mass lies in CO-dark gas, most of which is diffuse gas, poorlyshielded due to low dust column density. The CO-dark molecular gas tends to bedominated by [CII], though [CI] also serves as a bright tracer of the dark gasin many instances. At the same time, [CII] also tends to trace neutral atomicgas. As a result, when we quantify the conversion factors for the threecarbon-based tracers of molecular gas, we find that [CI] suffers the leastcontamination from diffuse atomic gas, and is relatively insensitive tosecondary parameters.

Dark Molecular Gas in Simulations of z~0 Disc Galaxies