Exploring magnetic fields on the largest scales

磁场弥漫着宇宙，包括星系团，在那里它们绵延数百万光年。最近，在所谓的“巨晕”星系团周围的更大尺度上发现了磁场。这些磁性巨晕之所以重要，既是因为它们的存在挑战了磁场起源的理论，也是因为它们可能在宇宙中大尺度结构的增长中发挥作用。传统的探测磁场方向的技术在这些巨大的尺度上是失败的。威斯康星大学麦迪逊分校的一个研究小组开发了一种新方法--梯度技术(GT)，用来测量巨晕中的磁场方向。GT通过利用受磁场影响的流体湍流运动的特性来揭示磁场方向。使用钱德拉X射线望远镜和低频阵列射电望远镜获得的磁场地图将用于(1)检验巨晕磁场起源的理论，以及(2)巨晕磁场在高能粒子加速和星系团内部结构发展中发挥作用的部分。学生，包括研究生和本科生，将接受将新技术应用于观测数据、分析观测数据和测试理论预测的培训。该团队将积极参与高中生参与的推广活动。该团队将使用广泛的数值模拟来衡量GT在测量磁场方面的准确性。银河集群合并目录中的多维数据集将用于此目的。数值模拟的结果将被用来更好地理解关键参数对由GT方法得出的磁场图的影响。这些因素包括等离子体磁化、湍流阿尔芬数、星系团的演化阶段和望远镜的分辨率。可以进行极化测量的星系团部分的GT磁图，即无线电遗迹，将与现有的极化数据进行比较。对合并和松弛星系团的晕和遗迹的磁场结构进行比较，以评估星系团合并的效果。分析了具有不同红移、不同动力学状态、有无冷核的团簇的磁结构，并与湍流发电机理论的理论预测进行了比较。根据获得的磁场结构，将重新评估宇宙线再加速和传播的影响。对冷核心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所获得的磁场数据进行比较。这一奖项反映了NSF的法定任务，并通过使用基金会的智力优势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.