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Limits on the Density of Intergalactic Ionized Hydrogen

Naturevolume 207pages736737 (1965) | Download Citation



OBSERVATIONS of background radiation at radio frequencies can be utilized to place limits on the amount of ionized hydrogen which exists in intergalactic space. For this purpose, I have calculated the intensity to be expected at low frequencies from cosmic sources in an Einstein–de Sitter universe. Intergalactic matter is assumed to possess the following properties: (1) The electron kinetic temperature is independent of time, as suggested by the form of the cosmic energy equation given by Layzer1. (2) Intergalactic matter is pure hydrogen (that is, no other source contributes electrons). (3) The electron density has attained its equilibrium value for specified temperature and density. (4) The electron temperature T > 104 °K (that is, hydrogen is ionized). (5) The electron temperature is much greater than the temperature appropriate to the radiation density. (6) The density of intergalactic hydrogen is a constant fraction, χ of the mean density, ρ¯, that is: where χ is 1 and independent of time.

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    Layzer, D., Astrophys. J., 138, 174 (1963); Ann. Rev. Astronomy and Astrophysics, 2, 341 (1964).

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    Layzer, D. (in preparation).

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  1. Harvard College Observatory, Cambridge, Massachusetts



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