Cool dark gas in Cygnus X: The first large-scale mapping of low-frequency carbon recombination lines

TL;DR

This study maps CO-dark gas traced by low-frequency carbon recombination lines in Cygnus X using the GBT, revealing arcs and filaments of cool C$^+$-bearing gas around CO-rich regions. By stacking 274–399 MHz CRRLs, the authors produce a 48.3′ resolution line-to-continuum data cube and show a strong link between CRRLs and the local FUV field, with $G_0$ estimated from 8 μm PAH emission in the range $40$–$165$ (median ~90). They find a mild positive correlation with $^{13}$CO (r ≈ 0.63, slope ≈ 0.46) and a characteristic peak separation of ~12 pc between CRRL and CO peaks, indicating CRRLs trace outer, CO-dark envelopes. Densities inferred for the C$^+$/H$_2$ layer span $n oughly 20$–$900$ cm$^{-3}$, with a turbulent, pressure-dominated evolution and an H$_2$ formation timescale of ~2.6 Myr, underscoring the power of low-frequency CRRLs to illuminate CO-dark gas and molecular cloud formation.

Abstract

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$α$ 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 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 $1.3 \pm 0.2$. 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$α$ 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 $n \approx 20 - 900$ cm$^{-3}$ with a nominal $n \approx 400$ 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.

Figures (13)

• Figure 1: RGB compilation in the Cygnus X region covering the footprint that we surveyed; in red is the CGPS 1.4 GHz continuum Taylor2003, green is MSX 8 $\mu$m PAH emission Schneider2006, and blue is $^{13}$CO (1-0) tracing (some of the) molecular gas Schneider2010. Left: The numbers enclosed by a circle in black indicate the 'DR' continuum sources identified in 5 GHz observations by Downes1966, which are mostly thermal H ii regions, except for DR3 and DR4 that make up the supernova remnant $\gamma$-Cygni. Dotted circles show the Cyg OB2 association and open clusters NGC 6910 and NGC 6913. The region at $\ell > 80$$^{\circ}$ commonly referred to as Cyg X North and at $\ell < 80$$^{\circ}$ (also typically at lower latitudes) as Cyg X South are also indicated. Right: Contours of velocity-integrated C273$\alpha$ (moment 0), drawn at $[3,5.5,8,...28]\sigma$. C273$\alpha$ is detected from 75% of the mapped region.
• Figure 2: Spatially-averaged line emission of the survey region. The CRRL 322 MHz temperature brightness spectrum of the effective C273$\alpha$ line, which is the spatial average over the entire mapped region. Spatially-averaged spectra of additional gas phases have been normalized to match the peak of C273$\alpha$; $^{12}$CO Dame2001 traces the molecular phase, H i 21 cm HI4PICollaboration2016 traces the atomic phase, and H273$\alpha$ (these data) traces diffuse ionized gas. We also show the non-detection of C$104\alpha$ from GDIGS 5.8 GHz observations Anderson2021, emphasizing the stimulated nature of the C273$\alpha$ 321.6 MHz observations.
• Figure 3: Channel maps of C273$\alpha$. Each channel map has been integrated over 2 channels, equivalent to 1 km s$^{-1}$. The central velocity of the map is shown in its top left corner. In the first channel, numbers indicate the DR identifications Downes1966; these sources are marked with a "+" in subsequent channel maps. The color bar, shown in the last channel, indicates the magnitude of $T_{\mathrm{C}273\alpha}/T_{\mathrm{C}}$ in units of 10$^{-3}$, and the color scale is the same for all maps. Gray contours are drawn at 3$\sigma$ and black contours are drawn at $[4, 6, 8, 10, 12, 14] \sigma$ where $\sigma_{\mathrm{median}} \approx 1.9 \times 10^{-4}$. The bottom right panel shows the C273$\alpha$ emission integrated over $-$13 to 17 km s$^{-1}$; black contours are $[4, 6, 8,... , 24] \sigma$.
• Figure 4: Aperture locations (salmon colored grid lines) and IDs (white number) for the extracted spectra shown in Figure \ref{['fig:specgrid_multiphase']}.
• Figure 5: Multi-phase spectra over the mapped region. C273$\alpha$ emission is dark blue, $^{13}$CO emission is in magenta, and H i 21 cm is green. The aperture ID is shown in a box in the upper-left corner of each spectrum. The y-axis on the right indicates the intensity in units of K for the $^{13}$CO spectra and for H i spectra that have been divided by a factor of 140.