Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254. II. Structures of magnetic fields
Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254. II. Structures of magnetic fields
复制标题
受星团影响的螺旋星系 NGC 4254 中的磁场和气体。 II。
DOI：
10.1051/0004-6361:20078688
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发表时间：
2007
影响因子：
6.5
通讯作者：
K. Chyży
中科院分区：
文献类型：
--
作者：
K. Chyży
目标。本文研究了室女座星系团螺旋星系NGC 4254中非对称射电偏振辐射的起源，并探讨了星系团环境对磁场性质的影响。方法.磁场的结构进行了分析与“磁地图”的概念，呈现不同的磁场分量（总，规则，和随机）在整个星系的分布，免费的法拉第旋转和投影效应。一些不同的物理现象影响的磁场建模分析和面临的星系的去极化模式和分布的磁场强度从多频偏振无线电观测。结果对NGC 4254内禀磁场矢量方向的研究表明，它们在整个星系中的剧烈变化（从0°到超过40°）不能仅仅由发电机过程产生，而必须由密度波和局部气体流动等效应主导。我们在星系内确定了总磁场强度与局部恒星形成率（SFR）之间的关系为幂律，指数为+0.18 ±0.01。我们发现磁场规则性与SFR（-0.32 ± 0.03）的关系是相反的，并认为这是由于在有活跃形成的恒星的区域中，随着湍流的增加，无规场的有效产生造成的。NGC 4254中的法拉第旋转测量值的分布表明，扰动的轴对称平均场发电机模式或轴对称和双对称模式的混合，规则场从盘向外定向，这与大多数观察到的星系相反。该星系的北方磁臂位于局域密度波的上游，其规则磁场强度约为8 μG，总磁场强度为17 μG，与在其他星系中观察到的磁场强度非常相似。但两个外臂（向密度波下游移动）内的磁场要强得多，规则场分量高达13 μG，总场高达20 μG。我们的集群对不同的磁场分量的影响的建模表明，在外部磁臂的电机感应磁场的拉伸和剪切力，而不是由集群冲压压力修改。这些力可能是由星系的引力相互作用触发的，它们产生了规则场的各向异性分量，并增强了偏振发射。我们还表明，在大臂间区域和星系的郊区的磁能超过气体热和湍流的能量，可能成为动态的重要。
Aims. The origin of asymmetric radio polarized emission in the Virgo Cluster spiral NGC4254 is investigated and the influence of cluster environment on the properties of magnetic fields is explored. Methods. Structures of magnetic fields are analyzed with the concept of "magnetic maps", presenting distributions of different magnetic field components (total, regular, and random) over the entire galaxy, free of Faraday rotation and projection effects. A number of different physical phenomena influencing the magnetic field are modeled analytically and confronted with the galaxy's depolarization pattern and distribution of magnetic field strength obtained from multifrequency polarimetric radio observations. Results. The study of orientation of intrinsic magnetic field vectors in NGC 4254 indicates that their dramatic variation (from 0° to more than 40°) throughout the galaxy cannot arise from the dynamo process alone, but must be dominated by effects such as density waves and local gas flows. We determine within the galaxy the relation between the strength of total magnetic field and the local star-formation rate (SFR) as a power-law with an index of +0.18 ±0.01. We find the opposite sense of the relation between magnetic field regularity and SFR (-0.32 ± 0.03), and suggest that it results from efficient production of random field with rising turbulence in the regions with actively-forming stars. The distribution of Faraday rotation measures in NGC 4254 indicates a perturbed axisymmetrical mean-field dynamo mode or a mixture of axisymmetrical and bisymmetrical ones with regular field directed outwards from the disk, which is contrary to most observed galaxies. The galaxy's northern magnetic arm, located on the upstream side of the local density wave, with regular field strength of about 8 μG and the total one of 17 μG, much resembles those observed in other galaxies. But the magnetic field within two outer arms (shifted downstream of a density wave) is much stronger, up to 13 μG in the regular field component and 20 μG in the total field. Our modeling of cluster influence on different magnetic field components indicates that within the outer magnetic arms the dynamo-induced magnetic fields are modified by stretching and shearing forces rather than by cluster ram pressure. Those forces, which are likely triggered by the galaxy's gravitational interaction, produce an anisotropic component of the regular field and enhance the polarized emission. We also show that the magnetic energy within the large interarm regions and the galaxy's outskirts exceeds the gas thermal and turbulent energy, likely becoming dynamically important.