Exploring magnetic fields on the largest scales

磁场遍布宇宙，包括在星系团中，它们延伸了数百万光年。 最近，在所谓的“megahalos”星系团周围发现了更大规模的磁场。这些磁巨晕很重要，因为它们的存在挑战了磁场起源的理论，还因为它们可能在宇宙大尺度结构的生长中发挥作用。传统的探测磁场方向的技术在这些巨大的尺度上失败了。 来自麦迪逊的威斯康星州大学的一个研究小组开发了一种新方法，即梯度技术（GT），用于测量巨晕中磁场的方向。GT揭示了磁场方向，通过采用流体湍流运动的性质，受到磁场的影响。利用钱德拉X射线望远镜和低频阵列射线射电望远镜获得的磁场图将用于：（1）检验巨晕磁场起源的理论;（2）巨晕磁场在高能粒子加速和星系团内结构发展中的作用。学生，研究生和本科生，将接受培训，将新技术应用于观测数据，分析观测和测试理论预测。该团队将积极参与涉及高中生的外展活动。该团队将采用广泛的数值模拟来衡量GT测量磁场的准确性。来自Galaxy Cluster Merger Catalog的立方体将用于此目的。数值模拟的结果将用于更好地理解关键参数对GT方法得出的磁场图的影响。这些因素包括等离子体的磁化强度、湍流阿尔芬数、星系团的演化阶段和望远镜的分辨率。 星系团中可能进行极化测量的部分的GT磁图，即，无线电遗迹，将与现有的极化数据进行比较。比较了团簇合并和弛豫时晕和遗迹中的磁场结构，以评价团簇合并的效果。我们将分析具有不同红移、不同动力学状态、不同冷核的团簇的磁结构，并将结果与湍流发电机理论的理论预测进行比较。宇宙射线再加速和传播的影响将根据所获得的磁场结构重新评估。冷核的热浮力不稳定性和星系团外围的磁热不稳定性的预测将与获得的磁场数据进行比较。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.