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arXiv 1502.05439astro-ph.GA
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附近螺旋星系IC 342的磁场:多频射电偏振研究

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Rainer Beck

摘要:

用Effelsberg和VLA望远镜在四个波段观测到IC 342的总射电和偏振射电连续辐射。同步辐射辐射的径向尺度长度的频率依赖性表明宇宙射线电子的能量依赖性传播,可能通过流不稳定性。总磁场的均分强度通常为15 μ G,有序场的均分强度为5 μ G。法拉第旋转的偏振角揭示了一个基本的规则场,只有约0.5 μ G的强度与大规模的轴对称螺旋图案,签名的平均场发电机,和一个约10倍强的字段,波动的规模为几个100 pc。IC 342中心区域棒星系周围的磁场类似于大型棒星系;它的规则螺旋磁场指向外,与盘中的磁场相反。圆盘中的偏振辐射集中在:(1)一条窄臂,宽约300皮秒差距,相对于东臂向内位移约200皮秒差距,表明磁场被密度波压缩;(2)在北旋臂周围具有300-500 pc宽的宽旋臂,其偏振发射,偏振角,法拉第旋转测量的尺度约为2kpc,表明帕克不稳定性产生了螺旋扭曲的通量管:(3)在西北部的臂间区域有一个初步的“磁臂”:(4)外星系有几个宽的旋臂,与总中性气体中的旋臂有关;(5)西南星系外围的短特征,可能因潮汐相互作用而扭曲。- 平均场发电机产生和发展“磁臂”可能需要一个在几个星系旋转周期内稳定的螺旋图案。IC 342中的发电机缓慢而微弱,可能受到沙洲、潮汐相互作用或瞬时螺旋模式的干扰。

英文摘要:

The total and polarized radio continuum emission of IC 342 was observed in four wavelength bands with the Effelsberg and VLA telescopes. The frequency dependence of the radial scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total magnetic field is typically 15 muG, that of the ordered field 5 muG. Faraday rotation of the polarization angles reveals an underlying regular field of only about 0.5 muG strength with a large-scale axisymmetric spiral pattern, signature of a mean-field dynamo, and an about 10x stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; its regular spiral field is directed outwards, opposite to that in the disk. The polarized emission in the disk is concentrated in: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary "magnetic arm" in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. - The generation and development of "magnetic arms" by a mean-field dynamo probably need a spiral pattern that is stable over a few galactic rotation periods. The dynamo in IC 342 is slow and weak, probably disturbed by the bar, tidal interaction, or a transient spiral pattern.

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