Radio synchrotron emission, its polarization and Faraday rotation of the polarization angle are powerful tools to study the strength and structure of magnetic fields in galaxies. Unpolarized synchrotron emission traces isotropic turbulent fields which are strongest in spiral arms and bars (20–30 $$\upmu $$μG) and in central starburst regions (50–100 $$\upmu $$μG). Such fields are dynamically important; they affect gas flows and drive gas inflows in central regions. Polarized emission traces ordered fields, which can be regular or anisotropic turbulent, where the latter originates from isotropic turbulent fields by the action of compression or shear. The strongest ordered fields (10–15 $$\upmu $$μG) are generally found in interarm regions. In galaxies with strong density waves, ordered fields are also observed at the inner edges of spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies and in central regions. Ordered fields in interacting galaxies have asymmetric distributions and are a tracer of past interactions between galaxies or with the intergalactic medium.—Faraday rotation measures of the diffuse polarized radio emission from galaxy disks reveal large-scale spiral patterns that can be described by the superposition of azimuthal modes; these are signatures of regular fields generated by mean-field dynamos. “Magnetic arms” between gaseous spiral arms may also be products of dynamo action, but need a stable spiral pattern to develop. Helically twisted field loops winding around spiral arms were found in two galaxies so far. Large-scale field reversals, like the one found in the Milky Way, could not yet be detected in external galaxies. In radio halos around edge-on galaxies, ordered magnetic fields with X-shaped patterns are observed. The origin and evolution of cosmic magnetic fields, in particular their first occurrence in young galaxies and their dynamical importance during galaxy evolution, will be studied with forthcoming radio telescopes like the Square Kilometre Array.

中文翻译：

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螺旋星系中的磁场

射电同步辐射、其极化和极化角的法拉第旋转是研究星系中磁场强度和结构的有力工具。非极化同步加速器发射追踪各向同性湍流场，这些湍流场在旋臂和螺旋棒（20-30 $\upmu $$μG）和中央星暴区域（50-100 $\upmu $$μG）中最强。这些字段在动态上很重要；它们影响天然气流动并推动中部地区的天然气流入。偏振发射跟踪有序场，可以是规则的或各向异性的湍流，后者在压缩或剪切作用下源自各向同性湍流场。最强的有序场（10-15 $$\upmu $$μG）通常出现在臂间区域。在具有强密度波的星系中，在旋臂的内边缘也观察到有序场。具有螺旋图案的有序场存在于宏伟设计的、棒状和絮状星系以及中心区域。相互作用星系中的有序场具有不对称分布，并且是星系之间或与星系间介质过去相互作用的示踪剂。——来自星系盘的漫射极化无线电发射的法拉第旋转测量揭示了大尺度螺旋模式，可以通过叠加来描述方位角模式；这些是由平均场发电机生成的常规场的签名。气态旋臂之间的“磁臂”也可能是发电机作用的产物，但需要稳定的螺旋模式才能发展。迄今为止，在两个星系中都发现了缠绕在旋臂上的螺旋扭曲场环。像在银河系中发现的那样的大规模场逆转，在外部星系中尚无法检测到。在边缘星系周围的射电晕中，可以观察到具有 X 形图案的有序磁场。宇宙磁场的起源和演化，特别是它们在年轻星系中的首次出现及其在星系演化过程中的动力学重要性，将用即将推出的射电望远镜（如平方公里阵列）进行研究。