Exploring magnetic fields on the largest scales

Magnetic fields permeate the cosmos, including in clusters of galaxies where they extend for millions of light years. Recently, magnetic fields were discovered on even larger scales around clusters of galaxies in so-called "megahalos". These magnetic megahalos are important both because their existence challenges theories of the origins of magnetic fields and because they may play a part in the growth of large-scale structure in the universe. Traditional techniques to probe the magnetic field direction fail on these gigantic scales. A team from University of Wisconsin, Madison, have developed a new method, the Gradient Technique (GT), to measure the direction of magnetic fields in megahalos. The GT reveals the magnetic field direction by employing the properties of fluid turbulent motions that are affected by the magnetic field. Maps of magnetic fields obtained using the Chandra X-ray telescope and the LOw Frequency ARray (LOFAR) radio telescope will be used to (1) test theories of the origin of the magnetic field in megahalos, and (2) the part megahalo magnetic fields play in the acceleration of energetic particles and the development of structures within galaxy clusters. Students, both graduate and undergraduates, will be trained in applying the new technique to observational data, analyzing observations and testing theoretical predictions. The team will be actively involved in outreach involving high school students. The team will employ extensive numerical simulations to gauge the accuracy of GT in measuring magnetic fields. Cubes from Galaxy Cluster Merger Catalog will be used for this purpose. The results of the numerical simulations will be used to better understand the effect of key parameters on maps of the magnetic field derived from the GT method. These include plasma magnetization, turbulent Alfven number, the evolutional stage of the cluster and the resolution of the telescope. The GT magnetic maps of the parts of clusters where polarization measurements are possible, i.e., radio relics, will be compared with the available polarization data. The structure of magnetic fields in halos and relics will be compared for merging and relaxed clusters to evaluate the effect of cluster merger. The magnetic structure of clusters with different redshifts, dynamical states, with/without cool cores will be analyzed with the results compared to the theoretical predictions of turbulent dynamo theory. The effects of cosmic ray re-acceleration and propagation will be re-evaluated in view of the obtained magnetic field structure. The predictions of the heat-buoyancy instability in cool cores and magnetothermal instability in outskirts of the galaxy clusters will be compared with the obtained magnetic field data. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.