2025ApJ...992..216E

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$^{\prime} $ (21 pc) resolution. Of the 30 deg² surveyed, we detect line-synthesized C273α emission from 24 deg² and produce the first large-area maps of low-frequency CRRLs, which likely originate in CO-dark molecular gas. The morphology of the C273α 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 C273α and the 8 μm 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, G₀ ≈ 40-160. We determine the typical angular separation between C273α and ¹³CO emission to be 12 pc. Velocity differences between C273α and ¹³CO 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⁺/H₂ layer. The evolution of the C273α gas seems to be dominated by turbulent pressure, with a characteristic timescale to form H₂ 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.