The two basic components of the interstellar neutral hydrogen and its relation with the galactic structure

Details The two basic components of the interstellar neutral hydrogen and its relation with the galactic structure The two basic components of the interstellar neutral hydrogen and its relation with the galactic structure

Jun 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975ap%26ss..35...67q&link_type=abstract

Astrophysics and Space Science, vol. 35, June 1975, p. 67-80.

Physics

4

Absorption Spectra, Galactic Structure, Interstellar Gas, Abundance, Brightness Temperature, Dark Matter, H Ii Regions, Microwave Emission, Solar Physics

Scientific paper

The two basic components of interstellar neutral hydrogen, cool dense clouds and the hotter tenous medium around them, are investigated using 21-cm absorption data from the Parkes survey. Values are obtained for the optical depth, velocity half-width, temperature and density of typical clouds near the galactic plane, and the pressure equilibrium condition is found to be compatible with these values if the element abundances in the interstellar medium are assumed to be below the normal solar abundance. This lack of heavy elements is shown to suggest accretion onto grains, which is consistent with the observed narrow concentration of dark matter on the galactic gas layer. Comparison with data on H II regions suggests that the cool clouds are somewhat denser and less extensive than such regions, while comparison with 21-cm emission data shows that the galactic cloud layer is only about one-quarter as thick as the hot gas layer. It is concluded that only the cool dark clouds are related to the typical spiral features of the galaxy.

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