Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

2. MOLECULAR MASS DETERMINATIONS

Since the H₂ molecule has no permanent electric dipole moment and the lowest quadrupole rotational transitions lie in the infrared, indirect techniques must be utilized to estimate the H₂ abundance in cool, dense clouds. A number of papers have discussed the use of CO as a tracer of the mass of molecular hydrogen (see, for example, Sanders et al 1984, Bloemen et al 1986, Dickman et al 1986, Young et al 1986a, Scoville & Sanders 1987, Maloney & Black 1988, Elmegreen 1989, Devereux & Young 1990b). In spite of the high optical depth of the 2.6 mm CO line, the emission is generally believed to trace the mass of giant molecular clouds (GMCs) in luminous galaxies. In this section, we briefly summarize the theoretical, empirical, and observational basis for the use of CO as a tracer of H₂.