Astrochemistry

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

By Keith Cowing
Status Report
astro-ph.GA
January 24, 2025
Filed under , , , , , , , , , , ,
Cold Dark Gas in Cygnus X: The First Large-scale Mapping of Low-frequency Carbon Recombination Lines
RGB compilation in the Cygnus X region covering the footprint that we surveyed; in red is the CGPS 1.4 GHz continuum (Taylor et al. 2003), green is MSX 8 µm PAH emission Schneider et al. (2006), and blue is 13CO (1-0) tracing (some of the) molecular gas (Schneider et al. 2010). Left: The numbers enclosed by a circle in black indicate the ‘DR’ continuum sources identified in 5 GHz observations by Downes & Rinehart (1966), which are mostly thermal H ii regions, except for DR3 and DR4 that make up the supernova remnant γ-Cygni. Dotted circles show the Cyg OB2 association and open clusters NGC 6910 and NGC 6913. The region at ℓ > 80◦ commonly referred to as Cyg X North and at ℓ < 80◦ (also typically at lower latitudes) as Cyg X South are also indicated. Right: Contours of velocity-integrated C273α (moment 0), drawn at [3, 5.5, 8, ...28]σ. C273α is detected from 75% of the mapped region. -- astro-ph.GA

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 21 pc (48′) resolution. Of the 30 deg^2 surveyed, we detect line-synthesized C273alpha emission from 24 deg^2 and produce the first large-area maps of low-frequency CRRLs.

The morphology of the C273alpha emission reveals arcs, ridges, and extended possibly sheet-like gas which are often on the outskirts of CO emission and likely transitioning from HI-to-H_2. The typical angular separation of C273alpha and 13CO emission is 12 pc, and we estimate C273alpha gas densities of n_H ~ 40 – 400 cm^3. The C273alpha line profiles are Gaussian and likely turbulent broadened, spanning a large range of FWHM from 2 to 20 km/s with a median of 10.6 km/s.

Mach numbers fall within 10–30. The turbulent timescale is relatively short, 2.6 Myr, and we deduce that the turbulent pressure likely dominates the evolution of the C273alpha gas. Velocity offsets between C273alpha and 13CO components are apparent throughout the region and have a typical value of 2.9 km/s.

Two regimes have emerged from the data: one regime in which C273alpha and 13CO are strongly related (at N_H ~ 4 x 10^21 cm^-2), and a second, in which C273alpha emits independently of the 13CO intensity. In the former regime, C273alpha may arise from the the envelopes of massive clouds (filaments), and in the latter, C273alpha emits from cold clumps in a more-diffuse mix of HI and H_2 gas.

Kimberly L. Emig, Pedro Salas, Loren D. Anderson, D. Anish Roshi, Lars Bonne, Alberto D. Bolatto, Isabelle A. Grenier, Rebecca C. Levy, Dylan J. Linville, Matteo Luisi, M. Riley Owens, J. Poojapriyatharsheni, Nicola Schneider, Luigi Tibaldo, Alexander G. G. M. Tielens, Stefanie K. Walch, Glenn J. White

Comments: 26 pages. 13 figures. submitted to ApJ. Comments welcome!
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2501.13838 [astro-ph.GA] (or arXiv:2501.13838v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2501.13838
Focus to learn more
Submission history
From: Kimberly Emig
[v1] Thu, 23 Jan 2025 17:05:36 UTC (21,336 KB)
https://arxiv.org/abs/2501.13838

Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻