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


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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Fig. 1: The 5 GHz total intensity radio contour of the gravitational lensing system CLASS B1152+199 overlaid on the HST F814W image.
Fig. 2: Faraday depth spectra of images A (red) and B (blue) of the gravitational lensing system CLASS B1152+199 computed using the rotation measure synthesis technique, followed by deconvolution using the RM-Clean algorithm50.
Fig. 3: Coherent magnetic field strength (|B c(r B)|) at the galacto-centric radius of image B (r B = 2.6 kpc) in the lensing galaxy.


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Authors and Affiliations



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.

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Correspondence to S. A. Mao.

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Mao, S.A., Carilli, C., Gaensler, B.M. et al. Detection of microgauss coherent magnetic fields in a galaxy five billion years ago. Nat Astron 1, 621–626 (2017).

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