arXiv 0710.35582007-10-18astro-ph

螺旋星系的磁场和动力学

Magnetic fields and the dynamics of spiral galaxies

C. L. Dobbs, D. J. Price

备注：: 17 pages, 14 figures, accepted by MNRAS
DOI：: 10.1111/j.1365-2966.2007.12591.x

摘要：

通过对螺旋势作用下的星系盘进行三维MHD模拟，研究了螺旋星系中磁场的动力学。最近的流体动力学模拟表明，形成臂间的马刺，以及螺旋臂分子云的ISM模型，包括一个冷HI阶段。我们发现在这些计算中加入磁场的主要作用是抑制盘中结构的形成。然而，如果包括冷相，则如果冷气体中存在$β\gtrsim 0.1$，则仍然存在毛刺和螺旋臂团块。两相计算的一个警告是，通过假设热气体中的$β\gtrsim 10$的均匀初始分布，强调需要具有更一致的初始条件和热力学的模型。我们的模拟，只有温暖的气体不显示这样的结构，无论磁场强度。此外，我们发现，一个冷HI阶段的引入自然会产生观察到的程度的无序的磁场，这是再次缺席模拟只使用温暖的气体。虽然全球磁场遵循大尺度气流，但磁场还包含由冷气体在通过螺旋激波期间引起的速度分散产生的基本随机分量。在没有任何冷气体的情况下，暖相的磁场除了在螺旋激波中被压缩之外，仍然保持相对良好的秩序。我们的研究结果提供了一个自然的解释所观察到的高比例的无序磁场在螺旋星系，因此，我们预测的相对强度的随机和有序组件的磁场在螺旋星系中观察到的将取决于螺旋冲击的动力学。

英文摘要：

We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D MHD simulations of galactic discs subject to a spiral potential. Recent hydrodynamic simulations have demonstrated the formation of inter-arm spurs as well as spiral arm molecular clouds provided the ISM model includes a cold HI phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if $β\gtrsim 0.1$ in the cold gas. A caveat to two phase calculations though is that by assuming a uniform initial distribution, $β\gtrsim 10$ in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength. Furthermore, we find that the introduction of a cold HI phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.

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