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arXiv 0801.3542astro-ph
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宇宙射线和附近星暴星系NGC 253的磁场

Cosmic rays and the magnetic field of the nearby starburst galaxy NGC 253

V. Heesen, R. -J. Dettmar, M. Krause, R. Beck

摘要:

利用射电偏振测量,我们研究了宇宙射线(CR)的运输,三维磁场结构,和其他ISM阶段的NGC 253晕的功能之间的联系。我们提出了一个新的敏感的无线电连续地图的NGC 253的组合VLA和Effelsberg观测在λ 6.2厘米。我们发现了一个显著的射电晕,其厚度约为1.7kpc。垂直连续辐射的局域尺度高度与宇宙线电子(CRE)寿命之间的线性关系要求垂直CRE体速度为270 km/s。NGC 253的磁场结构类似于一个“X”形的结构,其中大尺度磁场的方向仅在盘的内部区域和距离银河系中平面很小的距离处是平面平行的。在较大的银河系中心半径和远离中平面的垂直分量变得重要。这一点在原子核东南方的“射电尖峰”位置最为明显,那里的磁场方向几乎是垂直的。我们使用具有规定倾角和螺距角的螺旋磁场为主导环形(r,phi)磁场分量制作了一个简单的模型。通过从观测中减去模型而揭示的剩余极向(r,phi,z)磁场分量显示出以核为中心的明显的“X”形磁场取向。极向磁场的取向角与由垂直CR体速度和旋转速度的叠加描述的磁场传输一致。因此,我们提出了一个盘风的运输宇宙射线,磁场,和(部分)电离气体从磁盘到晕。

英文摘要:

Using radio polarimetry we study the connection between the transport of cosmic rays (CR's), the three-dimensional magnetic field structure, and features of other ISM phases in the halo of NGC 253. We present a new sensitive radio continuum map of NGC 253 obtained from combined VLA and Effelsberg observations at lambda 6.2 cm. We find a prominent radio halo with a scaleheight of the thick radio disk of 1.7 kpc. The linear dependence between the local scaleheight of the vertical continuum emission and the cosmic ray electron (CRE) lifetime requires a vertical CR bulk speed of 270 km s^-1. The magnetic field structure of NGC 253 resembles an ``X''-shaped configuration where the orientation of the large-scale magnetic field is plane-parallel only in the inner regions of the disk and at small distances from the galactic midplane. At larger galactocentric radii and further away from the midplane the vertical component becomes important. This is most clearly visible at the location of the ``radio spur'' southeast of the nucleus, where the magnetic field orientation is almost vertical. We made a simple model for the dominant toroidal (r,phi) magnetic field component using a spiral magnetic field with prescribed inclination and pitch angle. The residual poloidal (r,phi,z) magnetic field component which was revealed by subtracting the model from the observations shows a distinct ``X''-shaped magnetic field orientation centered on the nucleus. The orientation angle of the poloidal magnetic field is consistent with a magnetic field transport described by the superposition of the vertical CR bulk speed and the rotation velocity. Hence, we propose a disk wind which transports cosmic rays, magnetic field, and (partially) ionized gas from the disk into the halo.

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