Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990nascp3084....4f&link_type=abstract
In NASA, Ames Research Center, The Interstellar Medium in External Galaxies: Summaries of Contributed Papers p 4-5 (SEE N91-1410
Astronomy and Astrophysics
Astronomy
Brightness, Far Ultraviolet Radiation, Hydrogen, Interstellar Matter, Molecular Clouds, Neutral Gases, Spiral Galaxies, Stars, Atoms, Hypotheses, Permeating, Spaceborne Astronomy
Scientific paper
Shaya and Federman (1987) suggested that the ambient ultraviolet flux at 1000 A permeating a spiral galaxy controls the neutral hydrogen (HI) surface density in the galaxy. They found that the atomic envelopes surrounding small molecular clouds, because of their great number, provide the major contribution to the HI surface density over the stellar disk. The increase in HI surface density with later Hubble types was ascribed to the stronger UV fields from more high-mass stars in later Hubble types. These hypotheses are based on the observations of nearby diffuse interstellar clouds, which show a sharp atomic-to-molecular transition (Savage et al. 1977), and on the theoretical framework introduced by Federman, Glassgold, and Kwan (1979). Atomic envelopes around interstellar clouds in the solar neighborhood arise when a steady state is reached between photodissociation of H2 and the formation of H2 on grains. The photodissociation process involves photons with wavelengths between 912 A and 1108 A. Shaya and Federman used H-alpha flux as an approximate measure for the far UV flux and made their comparisons based on averages over Hubble type. Here, researchers compare, on an individual basis, UV data obtained with space-borne and balloon-borne instruments for galaxies with measurements of HI surface density (Warmels 1988a, b). The comparisons substantiate the conclusion of Shaya and Federman that the far UV field controls the HI content of spiral galaxies.
Federman Steven Robert
Ström C.
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