COLLISIONAL EXCITATION OF THE [C ii] FINE STRUCTURE TRANSITION IN INTERSTELLAR CLOUDS

By Paul F. Goldsmith, W. Langer, J. Pineda, Thirukumaran Velusamy. 2012

138 citations recorded by the source corpus.

We analyze the collisional excitation of the 158 μm (1900.5 GHz) fine structure transition of ionized carbon in terms of line intensities produced by simple cloud models. The single C+ fine structure transition is a very important coolant of the atomic interstellar medium (ISM) and of photon-dominated regions in which carbon is partially or completely in ionized form. The [C ii] line is widely used as a tracer of star formation in the Milky Way and other galaxies. Excitation of the [C ii] fine structure transition can be via collisions with hydrogen molecules, atoms, and electrons. Analysis of [C ii] observations is complicated by the fact that it is difficult to determine the optical depth of the line. We discuss the excitation of the [C ii] line, deriving analytic results for several limiting cases and carry out numerical solutions using a large velocity gradient model for a more inclusive analysis. For antenna temperatures up to 1/3 of the brightness temperature of the gas kinetic temperature, the antenna temperature is linearly proportional to the column density of C+ irrespective of the optical depth of the transition. This is appropriately referred to as the effectively optically thin approximation. We review the critical densities for excitation of the [C ii] line by various collision partners, briefly analyze C+ absorption, and conclude with a discussion of C+ cooling and how the considerations for line intensities affect the behavior of this important coolant of the ISM.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d21f300c81e3021d774eb3048d892404e4bf3a05

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In Astronomy and Astrophysics

1. C. Casey
2. Sylvain Veilleux
3. D. Narayanan
4. R. Maiolino
5. A. Cooray
6. D. Sanders
7. D. M. Alexander
8. Lee Armus

In Instrumentation

1. Sylvain Veilleux
2. D. Sanders
3. C. Casey
4. R. Maiolino
5. A. Cooray
6. S. Aalto
7. D. Narayanan
8. Lee Armus

Citations in the corpus

• Dusty Star Forming Galaxies at High Redshift (2014, background): “Beyond this, the critical densities for [CII] emission can range from ∼5–50 cm−3 for collisions with electrons, to ∼1–8 × 103 cm−3 for collisions with neutral atomic or molecular hydrogen (Goldsmith et al., 2012).”
• A Herschel [C ii] Galactic plane survey - I. The global distribution of ISM gas components (2013, background): “For optically thin emission, the [C ii] intensity (in units of K km s−1) is related to the C+ column density, NC+ (cm−2), and volume density of the collisional partner, n (e−, H, or H2; cm−3), as (see e.g. Goldsmith et al. 2012), I[CII] = NC+ [ 3.05 × 1015 ( 1 + 0.5 ( 1 + Aul Ruln ) e91.”
• A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies (2017, background): “…galaxies– is expected to arise from dense PDRs surrounding young, massive stars (Hollenbach & Tielens 1997), where the [C ii]158 is collisionally excited by neutral and molecular hydrogen, with ncrH ,[CII] ≃ 3.0× 103 cm−3 and ncrH2 ,[CII]≃ 6.1× 103 cm−3, at T =100K (Goldsmith et al. 2012).”
• Cool outflows in galaxies and their implications (2020, background): “In the case of optically thin emission and neglibible background, the intensity of the [C II] emission (I[CII], in erg s−1 cm−2 sr−1) relates to the temperature (T ), density (n), and column density (N) of the gas through (Crawford et al. 1985; Goldsmith et al. 2012)”