Abstract
The CO(1-0) transition is the most important molecular emission line for studying molecular gas in the ISM. Unfortunately, it is nearly always optically thick, and so determining N(CO) and, more importantly, N(H2) is complicated. Over the years several empirical techniques have been worked out for using the CO(1-0) line to determine the physically relevant quantity, N(H2). In this chapter we discuss these techniques with particular emphasis on determining the conversion factor (called X CO ) for diffuse and translucent molecular gas. The chapter closes with some comments on using the CO(1-0) line to trace dark gas.
Every story of conversion is a story of blessed defeat.
C.S. Lewis
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Notes
- 1.
Although the term is reminiscent of dark matter, dark gas is baryonic and has nothing to do with the former.
- 2.
Even the 305 m Arecibo radiotelescope has a beam size of ∼ 4’ at 1420 MHz.
- 3.
The separation of the fraction of far infrared emission that is associated with the target cloud from any unrelated foreground or background gas is effected spectroscopically. The velocity ranges can be chosen from the line profile measurements. We ignore the ionized component but this, in principle, can be included from X-ray observations and diffuse Hα emission.
- 4.
H. Bloemen and J.B.G.M. Bloemen are the same person.
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Magnani, L., Shore, S.N. (2017). The Relationship Between CO and H2 . In: A Dirty Window. Astrophysics and Space Science Library, vol 442. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-54350-4_8
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