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arXiv 1509.04522astro-ph.GA
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螺旋Galvanic中的磁场

Magnetic Fields in Spiral Galaxies

Rainer Beck

摘要:

射电同步辐射是研究星系中磁场强度和结构的有力工具。非偏振同步辐射追踪各向同性的湍流场,这些湍流场在旋臂和旋条(20-30 μG)和中心星爆区(50-100 μG)最强。这些气田在动力学上很重要;它们影响天然气流动,并驱动中部地区的天然气流入。--偏振发射跟踪有序场,其可以是规则的或各向异性的湍流,其中后者通过压缩或剪切作用源自各向同性的湍流场。最强的有序场(10-15 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。螺旋模式的有序场存在于大设计、棒星系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的分布,是过去星系之间或星系际介质相互作用的示踪物。在边缘星系周围的射电晕中,观测到了X形图案的有序磁场。--法拉第旋转测量星系盘的漫射偏振无线电辐射揭示了大规模的螺旋模式,可以通过方位角模式的叠加来描述;这些是平均场发电机产生的规则场的特征。气体旋臂之间的“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。到目前为止,在两个星系中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大规模场反转,还不能在外部星系中检测到。--将利用即将建成的平方公里阵列等射电望远镜研究宇宙磁场的起源和演变。

英文摘要:

Radio synchrotron emission is a powerful tool 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 μG) and in central starburst regions (50-100 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 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. In radio halos around edge-on galaxies, ordered magnetic fields with X-shaped patterns are observed. -- 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. -- The origin and evolution of cosmic magnetic fields will be studied with forthcoming radio telescopes like the Square Kilometre Array.

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