Abstract
The causes of the Faraday rotation in extragalactic radio sources have been reinvestigated in the light of new polarisation data which extend to large redshifts. Our results show convincingly that at nearly all galactic latitudes, the large rotation measures are generated outside of the Milky Way. Furthermore, any contribution from the intergalactic medium must be small when compared with the Faraday rotation generated at or in the extragalactic sources themselves. Having rotation measures for 108 QSOs of which 48 have redshift (z) greater than unity, we have been able to place the lowest and most reliable upper limits on any contribution from an intergalactic magneto-ionic medium at early cosmological epochs. The data show promise that it should soon be possible to ascertain whether or not the density of intergalactic ionised gas follows the cosmological scale factor.
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Kronberg, P., Simard-Normandin, M. New evidence on the origin of rotation measures in extragalactic radio sources. Nature 263, 653–656 (1976). https://doi.org/10.1038/263653a0
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DOI: https://doi.org/10.1038/263653a0
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