Letter | Published:

On Intergalactic Gas as a Possible Absorber of Extragalactic Radio Noise

Nature volume 204, pages 767768 (21 November 1964) | Download Citation

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Abstract

IT has recently been pointed out by Smith1 that most of the background radiation at low frequencies (< 10 Mc/s) is probably of extragalactic origin. The intensity of this radiation increases with decreasing frequency until about 3 Mc/s where it turns over sharply1–3. This turnover is probably due to absorption by ionized hydrogen, the required emission measure being about 4 cm−6 pc (for an electron temperature of 104 °K). It was suggested by Hoyle and Ellis4 that the ionized hydrogen lies in a slab parallel to the galactic plane, with an electron concentration ne (0.15 cm−3 and a thickness 200 pc. This model has been ruled out by the rocket and satellite observations of Walsh et al.3 and Smith1, which show that even when integrated over a hemisphere the radiation has a very sharp turnover in its spectrum, corresponding to the emission measure being nearly isotropic around us. These authors suggest5 that we are surrounded by a more or less spherical H II region with, say, ne 1 cm−3 and radius 4 pc (parameters which would not lead to unreasonably large Faraday rotation effects).

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References

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Affiliations

  1. Department of Applied Mathematics and Theoretical Physics, University of Cambridge.

    • D. W. SCIAMA

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https://doi.org/10.1038/204767a0

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