Determination in External Galaxies

For the reasons discussed in Section II1.A, molecular emission is a good tracer of H2. Since CO is the most abundant molecule after H2, it has become the de facto standard for deriving the molecular mass in galaxies. The molecular mass is derived from measurements of the CO emission by first measuring the CO luminosity from a molecular cloud in a galaxy  

FIGURE 11 CO emission contours shown superimposed on a Hubble Space Telescope image of M51. The CO image, which was constructed by combining 19 individual pointings of the Owens Valley Radio Observatory millimeter array, has a resolution of 2.3 . [From Aalto etal. (1999). Astrophysicai Journal 522, 165.] 

During the past five years morphological studies of spiral galaxies have been significantly advanced through two major millimeter interferometric surveys of the CO 1-0 

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