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 HI 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$\alpha$ 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$\alpha$ emission reveals arcs, ridges, and extended possibly sheet-like gas which are often on the outskirts of CO emission. We find a correlation between velocity-integrated C273$\alpha$ and the 8 $\mu$m intensity with a power-law slope of $1.3\pm 0.2$. We interpret the relation as the dependence of cool dark gas emission on the FUV radiation field, $G_0\approx 40-160$. We determine the typical angular separation between C273$\alpha$ and ${}^{13}$CO emission to be 12 pc. Velocity differences between C273$\alpha$ and ${}^{13}$CO are apparent throughout the region and have a typical value of 2.9 km s${}^{-1}$. We estimate gas densities of $n\approx 20-900$ cm${}^{-3}$ with a nominal $n\approx 400$ cm${}^{-3}$ in the C${}^{+}$/H${}_2$ layer. The evolution of the C273$\alpha$ 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.