Physics
Scientific paper
Nov 1964
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1964natur.204..767s&link_type=abstract
Nature, Volume 204, Issue 4960, pp. 767-768 (1964).
Physics
3
Scientific paper
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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