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I(CO)/N(H2) conversions and molecular gas abundances in spiral and irregular galaxies
Maloney, Philip; Black, John H.
AA(Arizona, University, Tucson, AZ), AB(Steward Observatory, Tucson, AZ)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 325, Feb. 1, 1988, p. 389-401. (ApJ Homepage)
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NASA/STI Keywords:
Interstellar Matter, Irregular Galaxies, Molecular Gases, Spiral Galaxies, Abundance, Carbon Monoxide, Milky Way Galaxy, Molecular Excitation
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Observations of emission in the J = 1-0 rotational transition of interstellar carbon monoxide have been used to infer column densities and masses of hydrogen on the basis of relations of the form N(H2)/ICO = alpha, where alpha is a constant. Although the large-scale value of alpha appears to be accurately constant over most of the Milky Way, the underlying reason for this is not fully understood, nor is it reasonable to expect similar behavior in other galaxies. The sensitivity of the conversion factor to such cloud parameters as density, temperature, and element abundances is examined quantitatively. It is found that conditions under which the conversion factor will be constant and equal to the Galactic value are almost certainly violated on global scales in many galaxies.

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