Astrochemistry

Extended Atomic Carbon Around Molecular Clouds

By Keith Cowing

Status Report

astro-ph.GA

March 12, 2025

Filed under APEX, astro-ph.GA, astrochemistry, astronomy, atomic carbon gas, Interstellar, Milky Way, Spectroscopy

Extended Atomic Carbon Around Molecular Clouds — Integrated maps of the observed strip. The left map shows the gas column density derived from the Hi-GAL observations. The circles there indicate the position and effective beam size of the spectra discussed in Fig. 4. The following maps give the line-integrated intensities of CO 2-1, ¹³CO 2-1, C¹⁸O 2-1, and [C i] 1-0 transitions. Colors represent the integrated intensity for the main velocity component from 11 to 17 km s⁻¹ , black and white contours the integral over the component from 18 to 22 km s⁻¹ , and orange and red contours the integral from 34 to 38 km s⁻¹ . The zero position in the plots is at R.A.(J2000)=7:10:39.8, Dec(J2000)=-11:27:09. — astro-ph.GA

Models predict that atomic carbon occurs at the surface and in the process of the formation of molecular clouds, making its fine structure transitions a diagnostic of cloud formation.

We study the distribution of atomic carbon in a small inconspicuous region towards the outer Galaxy that might be representative for a large fraction of the molecular gas of the Milky Way that is not directly affected by star formation. We observed a small strip of 5 arcminutes in the “Forgotten Quadrant”, the third quadrant of the Milky Way, with the APEX telescope in the 3P1−3P0 [CI] transition of atomic carbon and the J=2−1 transition of the three most abundant CO isotopologues and compared their distribution with existing measurements of gas column density and of ionized carbon.

The atomic carbon shows a very smooth distribution with the smallest gradient along the strip compared to the other lines. It is always brighter than ¹³CO and in one velocity-component even brighter than CO. In contrast to observations of many star-forming regions, the [CI] emission seems to extend beyond the molecular gas, in line with the models of photon-dominated regions (PDRs).

However, a standard PDR model fit to the observations fails because the models either predict more molecular gas, traced through C¹⁸O, or more diffuse gas, traced through [CII], than observed. The carbon-budget in the gas phase does not add up to the same column seen through dust emission.

To understand the [CI] emission from galaxies it is necessary to get the full statistics for the quiescent gas outside of the star-forming regions that behaves significantly different from dense gas exposed to high ultraviolet fields.

V. Ossenkopf-Okada, A. Karska, M. Benedettini, D. Colombo, R. Simon

Comments: Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2503.04967 [astro-ph.GA] (or arXiv:2503.04967v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2503.04967
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Submission history
From: Volker Ossenkopf-Okada
[v1] Thu, 6 Mar 2025 20:59:43 UTC (466 KB)
https://arxiv.org/abs/2503.04967
Astrobiology, Astrochemistry,

Keith Cowing

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) 🖖🏻

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