用SKA检验星系的磁场模型
Testing the magnetic field models of galaxies with the SKA
摘要：
未来的新一代射电望远镜SKA（平方公里阵列）及其前身将提供数量迅速增长的极化射电源。通过法拉第旋转测量法（RM），可以测量到数十万个朝向附近星系的偏振背景源，从而可以绘制详细的磁场图。我们的目标是估计所需的密度与SKA检测到的背景偏振源的可靠识别和重建附近的螺旋星系的磁场结构。我们建立了一个星系模型，其中包括电离气体和磁场图案的不同方位角对称（轴对称（ASS），双对称（BSS）和四对称螺旋（QSS），和叠加）加上一个晕磁场。RM波动与Kolmogorov谱由于湍流场和/或电离气体密度的波动叠加。磁结构的识别是可能的，从RM到星系背后的背景源或从星系本身的漫射偏振发射获得的连续RM图。在理想的条件下，大约几十个偏振源就足以进行可靠的识别。对于大量的背景源，无需预知就可以重建场结构。一个可靠的重建场结构需要至少20 RM值的削减沿投影短轴，翻译约1200源朝向星系。在低频工作的射电望远镜（LOFAR、ASKAP和低频SKA阵列）也可能是有用的仪器，如果背景源在低频仍然明显极化（略），则可在RM的帮助下进行现场识别或重建。
英文摘要：
The future new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Hundred and thousands polarized background sources can be measured towards nearby galaxies thus allowing their detailed magnetic field mapping by means of Faraday rotation measures (RM). We aim to estimate the required density of the background polarized sources detected with the SKA for reliable recognition and reconstruction of the magnetic field structure in nearby spiral galaxies. We construct a galaxy model which includes the ionized gas and magnetic field patterns of different azimuthal symmetry (axisymmetric (ASS), bisymmetric (BSS) and quadrisymmetric spiral (QSS), 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. Recognition of magnetic structures is possible from RM towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. Under favourite conditions, about a few dozens of polarized sources are sufficient for a reliable recognition. Reconstruction of the field structure without precognition becomes possible for a large number of background sources. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis which translates to approximately 1200 sources towards the galaxy. Radio telescopes operating at low frequencies (LOFAR, ASKAP and the low-frequency SKA array) may also be useful instruments for field recognition or reconstruction with the help of RM, if background sources are still significantly polarized at low frequencies (abriged).
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