The magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars1. The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, that is, Faraday rotation, yield an average value for the magnetic field of B ≈ 3 μG (ref. 2). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars3 suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain. Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z = 0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy4. This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past rather than stronger5.
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We wish to thank F. H. Shu for suggesting the merger model and H.-W. Chen for providing us with her reanalysed images of 3C 286. We also thank F. H. Shu, E. Gawiser and A. Lazarian for comments and the US National Science Foundation for financial support. The GBT is one of the facilities of the National Radio Astronomy Observatory, which is a center of the National Science Foundation operated under cooperative agreement by Associated Observatories, Inc. A.M.W., R.A.J. and J.X.P. are Visiting Astronomers at the W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California, the California Institute of Technology and the National Aeronautics and Space Administration.
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Wolfe, A., Jorgenson, R., Robishaw, T. et al. An 84-μG magnetic field in a galaxy at redshift z = 0.692. Nature 455, 638–640 (2008). https://doi.org/10.1038/nature07264
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