Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies

Abstract

The H 2 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 μ m and [C ii ] 158 μ m 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 H 2 abundances and CO ( J  = 1−0), [C i ] 609 μ m, and [C ii ] 158 μ m 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., H 2 gas with W CO  < 0.1 K km s −1 , we find that a significant amount (≳50%) of the total H 2 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.