Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA
Magnetic field structures of galaxies derived from analysis of Faraday rotation measures, and perspectives for the SKA
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从法拉第旋转测量分析得出的星系磁场结构以及 SKA 的前景
DOI：
10.1051/0004-6361:20078678
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发表时间：
2007
影响因子：
6.5
通讯作者：
M. Krause
中科院分区：
文献类型：
--
作者：
R. Stepanov;T. Arshakian;R. Beck;P. Frick;M. Krause
上下文。即将到来的新一代射电望远镜SKA（平方公里阵列）及其前身将提供快速增长的极化射电源。目标。我们的分析着眼于从这些来源中可以了解到的关于外部星系磁场的结构和演化的信息。方法。通过对星系背后背景源的法拉第旋转测量（RM）或从星系本身的漫射极化发射获得的连续RM图，可以识别磁性结构。我们建立了不同方位对称（轴对称、双对称、四对称螺旋和叠加）外加晕磁场的电离气体和磁场模式模型。由于湍流场和/或电离气体密度的波动，具有Kolmogorov谱的RM波动叠加在一起。假设偏振光源的外推数密度计数，我们在星系的立体角内生成了RM值的样本。应用各种模板，我们推导出最小数量的背景源和最小质量的观测。对于大量的信息源，在没有预知的情况下重构场结构成为可能。结果。借助$\chi^2$准则，可以从RM数据中清楚地识别出磁场的任何大规模规则分量。在最理想的条件下，几十个极化光源就足以获得可靠的结果。具有垂直分量的光晕场不影响识别结果。对于星系盘的小倾角和较大的RM湍流，所需的源数增加。通过更灵敏的观测可以克服源的平坦数密度分布。将该识别方法应用于m31附近区域的可用RM数据表明，背景源或银河系前景对RM有显著贡献。一个可靠的油田结构重建需要沿着投影的小轴至少20个RM值。结论。从极化背景源的RM数据中识别或重建规则场结构是未来射电望远镜的有力工具。用SKA测量频率在1ghz左右的RM，可以在距离约100mpc的星系中识别简单的场结构，并将允许测试发电机与原始或其他场起源模型。低频SKA阵列和LOFAR等低频前驱望远镜如果背景源在低频仍然明显极化，也可能具有良好的RM灵敏度。
Context. The forthcoming new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Aims. Our analysis looks at what can be learned from these sources concerning the structure and evolution of magnetic fields of external galaxies. Methods. Recognition of magnetic structures is possible from Faraday rotation measures (RM) towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. We constructed models for the ionized gas and magnetic field patterns of different azimuthal symmetries (axisymmetric, bisymmetric and quadrisymmetric spirals, and superpositions) plus a halo magnetic field. RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Assuming extrapolated number density counts of polarized sources, we generated a sample of RM values within the solid angle of the galaxy. Applying various templates, we derived the minimum number of background sources and the minimum quality of the observations. For a large number of sources, reconstruction of the field structure without precognition becomes possible. Results. Any large-scale regular component of the magnetic field can be clearly recognized from RM data with the help of the $\chi^2$ criterium. Under favorite conditions, a few dozen polarized sources are enough for a reliable result. A halo field with a vertical component does not affect the results of recognition. The required source number increases for small inclinations of the galaxy's disk and for larger RM turbulence. A flat number density distribution of the sources can be overcome by more sensitive observations. Application of the recognition method to the available RM data in the region around M 31 indicates that there are significant RM contributions intrinsic to the background sources or due to the foreground of the Milky Way. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis. Conclusions. Recognition or reconstruction of regular field structures from the RM data of polarized background sources is a powerful tool for future radio telescopes. Measuring RM at frequencies around 1 GHz with the SKA, simple field structures can be recognized in galaxies up to about 100 Mpc distance and will allow to test dynamo against primordial or other models of field origin. The low-frequency SKA array and low-frequency precursor telescopes like LOFAR may also have good RM sensitivity if background sources are still significantly polarized at low frequencies.