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arXiv 0809.0419astro-ph
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M33的高分辨率无线电连续测量:III。磁场

High-resolution radio continuum survey of M33: III. Magnetic fields

F. S. Tabatabaei, M. Krause, A. Fletcher, R. Beck

摘要:

利用3.6,6.2和20厘米处的线偏振强度和偏振角数据,我们确定了附近星系M33的法拉第旋转和去偏振的变化。规则磁场的3-D模型拟合到所观察到的极化角的方位角分布。法拉第旋转,在3.6和6.2厘米之间测量,线性分辨率为0.7千秒差距,显示出更多的变化在南部比北部的星系。M33在3.6厘米处的非热辐射中约有10%是偏振的。我们估计M33的平均总磁场强度和规则磁场强度分别为~ 6.4和2.5 $μ$ G。假设M33的圆盘是平坦的,规则的磁场由水平和垂直分量组成:然而,推断的垂直场可能部分是由于银河系的弯曲。水平场由半径为1至3 kpc的轴对称(m=0)模和半径为3至5 kpc的轴对称和双对称(m=0+1)模的叠加表示。在南半部分的差分法拉第旋转与强大的法拉第色散在南部的螺旋臂过剩似乎是负责观察到的波长相关的去极化的南北不对称。在每个环中存在轴对称m=0模式的规则磁场,这表明M33中有一个银河发电机在运转。螺旋规则磁场的螺距角通常小于光学螺旋臂的螺距角,但是是基于平均场发电机理论和M33的旋转曲线的简单估计的两倍大。湍流和磁能密度的均分表明M33中湍流气体运动产生的星际磁场。

英文摘要:

Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across the nearby galaxy M33. A 3-D model of the regular magnetic field is fitted to the observed azimuthal distribution of polarization angles. Faraday rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc, shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5 $μ$G, respectively. Under the assumption that the disk of M33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3 to 5 kpc radius. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m=0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M33's rotation curve. Generation of interstellar magnetic fields from turbulent gas motions in M33 is indicated by the equipartition of turbulent and magnetic energy densities.

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