Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984apj...281l...5w&link_type=abstract
Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X), vol. 281, June 1, 1984, p. L5-L8.
Astronomy and Astrophysics
Astronomy
19
Centimeter Waves, Hydrogen Atoms, Intergalactic Media, Radio Sources (Astronomy), Rarefied Gases, Relic Radiation, Spin Temperature, Electron Spin, Ionizing Radiation, Lyman Alpha Radiation
Scientific paper
The spin temperature of atomic hydrogen is considered for the very low density gas which is observed at the peripheries of galaxies and, recently, in the form of an intergalactic cloud. It is found that the scattering of Lyman-alpha radiation probably causes the spin temperature to be sufficiently different from the 3 K background and that application of the 'standard' (independent of spin temperature) interpretation for the mass of gas causing optically thin, 21 cm emission introduces negligible error. A seemingly minimal estimate for the flux of Lyman-alpha is obtained from the ionizations caused by the extragalactic X-ray flux found by continuing the observed power-law spectrum at 1-10 keV down to 100 eV. The possibility is mentioned that an independent determination of the spin temperature would provide new information about the extreme ultraviolet/soft X-ray flux outside of galaxies.
Deguchi Shigeki
Watson William D.
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