Hydrogen Maps the Galaxy

Fig. 4. Superimposed maps of the neutral hydrogen distribution in the Galaxy as interpreted by Kerr and by Weaver. The points marked L denote regions that Kerr interprets as markedly deficient in neutral hydrogen. Dashed lines and hatched areas indicate where the location is uncertain, not regions where the hydrogen is weak or strong. The radius of the solar circle is 10 kpc. Galactic longitude is shown around the edge of the map...

Molecular hydrogen is the dominant molecule the second most abundant molecule, CO, is four orders of magnitude less abundant. But H2 has no strong transitions in the microwave regions, CO is mainly used to map interstellar clouds in our galaxy and others, and also in quasars. The observation of... 

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