Extended carbon line emission in the galaxy: Searching for dark molecular gas along the G328 sightline

Journal Article · Sun Sep 20 00:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/811/1/13· OSTI ID:22882638

Burton, Michael G.; Ashley, Michael C. B.; Braiding, Catherine; Freeman, Matthew ^[1]; Kulesa, Craig ^[2]; Wolfire, Mark G. ^[3]; Hollenbach, David J. ^[4]; Rowell, Gavin; Lau, James ^[5]

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1. School of Physics, University of New South Wales, Sydney, NSW 2052 (Australia)
2. Steward Observatory, The University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States)
3. Astronomy Department, University of Maryland, College Park, MD 20742 (United States)
4. Carl Sagan Center, SETI Institute, 189 Bernado Avenue, Mountain View, CA 94043-5203 (United States)
5. School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005 (Australia)

We present spectral data cubes of the [CI] 809 GHz, {sup 12}CO 115 GHz, {sup 13}CO 110 GHz, and HI 1.4 GHz line emission from a ∼1 square degree region along the l=328{sup ∘} (G328) sightline in the Galactic Plane. Emission arises principally from gas in three spiral arm crossings along the sightline. The distribution of emission in the CO and [CI] lines is found to be similar, with the [CI] slightly more extended, and both are enveloped in extensive HI. Spectral line ratios per voxel in the data cubes are found to be similar across the entire extent of the Galaxy. However, toward the edges of the molecular clouds the [CI]/{sup 13}CO and {sup 12}CO/{sup 13}CO line ratios rise by ∼50%, and the [CI]/HI ratio falls by ∼10%. We attribute this to these sightlines passing predominantly through the surfaces of photodissociation regions (PDRs), where the carbon is found mainly as C or C{sup +}, while the H{sub 2} is mostly molecular, and the proportion of atomic gas also increases. We undertake modeling of the PDR emission from low density molecular clouds excited by average interstellar radiation fields and cosmic-ray ionization to quantify this comparison, finding that depletion of sulfur and reduced PAH abundance is needed to match line fluxes and ratios. Roughly one-third of the molecular gas along the sightline is found to be associated with this surface region, where the carbon is largely not to be found in CO. Approximately 10% of the atomic hydrogen along the sightline is cold gas within PDRs.

• OSTI ID: 22882638
• Journal Information: Astrophysical Journal, Vol. 811, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
CARBON
CARBON 12
CARBON 13
CARBON IONS
CARBON MONOXIDE
COMPARATIVE EVALUATIONS
COMPUTERIZED SIMULATION
CONCENTRATION RATIO
COSMIC RADIATION
DISTRIBUTION
ELEMENT ABUNDANCE
EMISSION
GALACTIC EVOLUTION
GALAXIES
HYDROGEN
IONIZATION
PHOTOLYSIS
POLYCYCLIC AROMATIC HYDROCARBONS
TELESCOPES