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Detection of microgauss coherent magnetic fields in a galaxy five billion years ago

Nature Astronomyvolume 1pages621626 (2017) | Download Citation


Magnetic fields play a pivotal role in the physics of interstellar medium in galaxies1, but there are few observational constraints on how they evolve across cosmic time2,3,4,5,6,7. Spatially resolved synchrotron polarization maps at radio wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies1,8,9 that are believed to grow from a seed field via a dynamo effect10,11. To directly test and characterize this theory requires magnetic field strength and geometry measurements in cosmologically distant galaxies, which are challenging to obtain due to the limited sensitivity and angular resolution of current radio telescopes. Here, we report the cleanest measurements yet of magnetic fields in a galaxy beyond the local volume, free of the systematics traditional techniques would encounter. By exploiting the scenario where the polarized radio emission from a background source is gravitationally lensed by a foreground galaxy at z = 0.439 using broadband radio polarization data, we detected coherent μG magnetic fields in the lensing disk galaxy as seen 4.6 Gyr ago, with similar strength and geometry to local volume galaxies. This is the highest redshift galaxy whose observed coherent magnetic field property is compatible with a mean-field dynamo origin.

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Author information


  1. Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, Bonn, D-53121, Germany

    • S. A. Mao
    • , O. Wucknitz
    • , A. Basu
    •  & R. Beck
  2. National Radio Astronomy Observatory, PO Box O, Socorro, NM, 87801, USA

    • C. Carilli
  3. Cavendish Astrophysics Group, Cambridge, CB3 0HE, UK

    • C. Carilli
  4. Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Toronto, ON, M5S 3H4, Canada

    • B. M. Gaensler
  5. Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, 08854, USA

    • C. Keeton
  6. Department of Physics, University of Toronto, Toronto, ON, M5S 1A7, Canada

    • P. P. Kronberg
  7. Department of Astronomy, The University of Wisconsin, Madison, WI, 53706, USA

    • E. Zweibel
  8. Department of Physics, The University of Wisconsin, Madison, WI, 53706, USA

    • E. Zweibel


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S.A.M. led the VLA proposal and observations, performed the data reduction, analysis and interpretation, and wrote the paper. C.C. and B.M.G. contributed to the VLA proposal and interpretation of the data. O.W. and C.K. contributed to the interpretation of the data from the lensing perspective. P.P.K. and E.Z. contributed to the VLA proposal. A.B. and R.B. contributed to the interpretation of the data. All authors discussed the results, interpretations and presentation of the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. A. Mao.

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