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Detecting the orientation of magnetic fields in galaxy clusters

Abstract

Clusters of galaxies, filled with hot magnetized plasma, are the largest bound objects in existence and an important touchstone in understanding the formation of structures in our Universe. In such clusters, thermal conduction follows field lines, so magnetic fields strongly shape the cluster’s thermal history; that some have not since cooled and collapsed is a mystery. In a seemingly unrelated puzzle, recent observations of Virgo cluster spiral galaxies imply ridges of strong, coherent magnetic fields offset from their centre. Here we demonstrate, using three-dimensional magnetohydrodynamical simulations, that such ridges are easily explained by galaxies sweeping up field lines as they orbit inside the cluster. This magnetic drape is then lit up with cosmic rays from the galaxies’ stars, generating coherent polarized emission at the galaxies’ leading edges. This immediately presents a technique for probing local orientations and characteristic length scales of cluster magnetic fields. The first application of this technique, mapping the field of the Virgo cluster, gives a startling result: outside a central region, the magnetic field is preferentially oriented radially as predicted by the magnetothermal instability. Our results strongly suggest a mechanism for maintaining some clusters in a ‘non-cooling-core’ state.

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Figure 1: Polarized radio ridges in comparison with the HI emission.
Figure 2: Simulations of spiral galaxies interacting with a uniform cluster magnetic field.
Figure 3: Streamlines in the rest frame of the galaxy.
Figure 4: Simulations of spiral galaxies interacting with a non-uniform cluster magnetic field.
Figure 5: Polarized radio emission: observations versus simulation.
Figure 6: Mapping out the magnetic field orientations in the Virgo galaxy cluster as inferred from the polarized synchrotron emission ridges of cluster spirals.

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Acknowledgements

The authors wish to thank A. Chung, B. Vollmer and M. Weżgowiec for providing observational data and acknowledge C. Thompson, Y. Lithwick, J. Sievers and M. Ruszkowski for discussions during the preparation of this manuscript. C.P. gratefully acknowledges the financial support of the National Science and Engineering Research Council of Canada. Computations were carried out on the GPC supercomputer at the SciNet HPC Consortium. SciNet is funded by: the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund—Research Excellence; and the University of Toronto. 3D renderings were carried out with Paraview.

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Contributions

C.P. initiated this project, carried out the analytic estimates, developed an approach for comparing to observations, presented and analysed the observational data, measured magnetic field angles and discussed their uncertainties and explored consequences for cluster physics. L.J.D. carried out magnetohydrodynamical simulations with Athena, explored the parameter space and visualized the simulations. Both authors contributed to the exploratory simulations with Flash, the development of the synchrotron polarization model and post-processing, the statistical analysis, the interpretation of the results and writing of the paper.

Corresponding author

Correspondence to Christoph Pfrommer.

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The authors declare no competing financial interests.

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Pfrommer, C., Dursi, J. Detecting the orientation of magnetic fields in galaxy clusters. Nature Phys 6, 520–526 (2010). https://doi.org/10.1038/nphys1657

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