Magnetic fields are an important component of the interstellar medium (ISM) and exhibit strongly varying field strengths and a non-trivial correlation with the gas density. Its dynamical impact varies between individual regions of the ISM and correlates with the orientation of the field with respect to the gas structures. Using high-resolution magneto-hydrodynamical simulations of the ISM, we explore the connection between the orientation of the field and the dynamical state of the gas. We find that the onset of gravitational instability in molecular gas above a density of $\rho \sim 10^{-21}\, \mathrm{g\, cm}^{-3}$$(n\sim 400\, \mathrm{cm}^{-3})$ coincides with an alignment of the magnetic field lines and the gas flow. At this transition, the gradient of the density changes from mainly perpendicular to preferentially parallel to the field lines. A connection between the three-dimensional alignment and projected two-dimensional observables is non-trivial, because of a large dispersion of the magnetic field orientation along the line of sight. The turbulent correlation lengths can be small compared to the typical integration lengths. As a consequence, the small-scale signal of the orientation can sensitively depend on the line of sight or the dynamical state of the cloud can fluctuate stochastically or be completely averaged out. With higher spatial resolution more small-scale structures are resolved, which aggravates the link between magneto-hydrodynamical quantities and projected observables.

中文翻译：

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恒星形成区域的磁场排列以及为什么它可能难以观察

磁场是星际介质 (ISM) 的重要组成部分，表现出强烈变化的场强以及与气体密度的非平凡相关性。它的动态影响在 ISM 的各个区域之间变化，并且与场相对于气体结构的方向相关。使用 ISM 的高分辨率磁流体动力学模拟，我们探索了场方向与气体动态状态之间的联系。我们发现，密度大于 $\rho \sim 10^{-21}\, \mathrm{g\, cm}^{-3}$$(n\sim 400\, \mathrm{cm}^{-3})$ 与磁场线和气流的对齐一致。在这个转变中，密度梯度从主要垂直于场线变为优先平行于场线。三维排列和投影的二维观测之间的联系是不平凡的，因为沿视线的磁场方向有很大的分散性。与典型的积分长度相比，湍流相关长度可以很小。因此，方向的小尺度信号可以敏感地依赖于视线，或者云的动态状态可以随机波动或完全平均。随着更高的空间分辨率，更多的小尺度结构被解析，这加剧了磁流体动力学量和投影可观测物之间的联系。与典型的积分长度相比，湍流相关长度可以很小。因此，方向的小尺度信号可以敏感地依赖于视线，或者云的动态状态可以随机波动或完全平均。随着更高的空间分辨率，更多的小尺度结构被解析，这加剧了磁流体动力学量和投影可观测物之间的联系。与典型的积分长度相比，湍流相关长度可以很小。因此，方向的小尺度信号可以敏感地依赖于视线，或者云的动态状态可以随机波动或完全平均。随着更高的空间分辨率，更多的小尺度结构被解析，这加剧了磁流体动力学量和投影可观测物之间的联系。