Fig 17.13.

Large-scale distribution of CO emission in the galaxy M3I.This map was made using a 1 m diameter telescope at the Center for Astrophysics (Harvard). [Thomas Dame, CFA/Dame,T et al., Astrophys. J., 4 1 8,730, 1993, Fig. 1(a)]

also be significant amounts of molecular gas in the outer parts of spiral galaxies.

In studies of several spirals it is found that the relative amounts of molecular and atomic hydrogen vary significantly. These variations occur both

Fig 17.14.

IRAS map of M3I.[NASA]

within a galaxy and from galaxy to galaxy. Within a galaxy, the general trend is to have the molecular hydrogen abundance fall off faster with radius than the atomic hydrogen abundance. We find some galaxies in which the molecular hydrogen makes up over half of the interstellar medium, and others in which it seems to be less than 10%, as determined from a deficiency of CO emission. In galaxies that seem to be deficient in molecular hydrogen, we still don't have observations with sufficient resolution to tell us whether this is because they contain fewer molecular clouds than the other galaxies, or whether the clouds are less dense. Also, even galaxies that appear to be deficient in molecular clouds have O stars. This tells us that we still do not fully understand the connection between molecular clouds and massive star formation.

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