Cool Dark Gas in Cygnus X: The First Large-scale Mapping of Low-frequency Carbon Recombination Lines

Understanding the transition from atomic gas to molecular gas is critical to explain the formation and evolution of molecular clouds. However, the gas phases involved, cold H i and CO-dark molecular gas, are challenging to directly observe and physically characterize. We observed the Cygnus X star-forming complex in carbon radio recombination lines (CRRLs) at 274–399 MHz with the Green Bank Telescope at 48 ′ (21 pc) resolution. Of the 30 deg 2 surveyed, we detect line-synthesized C273 α emission from 24 deg 2 and produce the first large-area maps of low-frequency CRRLs, which likely originate in CO-dark molecular gas. The morphology of the C273 α emission reveals arcs, ridges, and extended possibly sheet-like gas, which are often found on the outskirts of CO emission. We find a correlation between velocity-integrated C273 α and the 8 μ m intensity with a power-law slope of 1.3 ± 0.2. We interpret the relation as the dependence of cool dark gas emission on the far-ultraviolet radiation field, G 0 ≈ 40−160. We determine the typical angular separation between C273 α and 13 CO emission to be 12 pc. Velocity differences between C273 α and 13 CO are apparent throughout the region and have a typical value of 2.9 km s −1 . We estimate gas densities of n ≈ 20−900 cm −3 with a nominal n ≈ 400 cm −3 in the C + /H 2 layer. The evolution of the C273 α gas seems to be dominated by turbulent pressure, with a characteristic timescale to form H 2 of about 2.6 Myr. These observations underline the richness of low-frequency CRRLs to provide revelatory insights into the characteristics of (CO-)dark gas and the evolution of molecular gas.