1. Department of Physics and Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, California 92093-0424, USA
2. Astronomy Department, University of California, Berkeley, California 94720-3411, USA
3. UCO-Lick Observatory; University of California, Santa Cruz, Santa Cruz, California 95464, USA

Correspondence to: Arthur M. Wolfe¹ Correspondence and requests for materials should be addressed to A.M.W. (Email: awolfe@ucsd.edu).

Abstract

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 G (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 G in a galaxy at z = 0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 G 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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