具有各向异性热传导和绝热宇宙射线的热不稳定性：对星系团中冷丝的启示
Thermal Instability with Anisotropic Thermal Conduction and Adiabatic Cosmic Rays: Implications for Cold Filaments in Galaxy Clusters
摘要：
对附近星系团核心的观测显示出H $α$和分子发射线细丝。我们认为这是{ \em局部}热不稳定性在一个{ \em全局}稳定的星系团核心的结果。我们提出了本地，高分辨率，二维磁流体动力学模拟的热不稳定性的条件下，适当的内集群介质（ICM），模拟包括沿磁力线和绝热宇宙射线的热传导。热传导抑制沿磁力线的热不稳定性，其尺度小于场长度（热扩散ICM的$\gtrsim$ 10 kpc）。我们表明，在冷介质中的字段长度必须解决沿和垂直于磁场，以获得数值收敛的结果。由于垂直于磁场的传导可以忽略不计，热不稳定性导致在垂直方向上的精细尺度结构。因此，沿着磁力线的冷气体细丝是各向异性热传导的热不稳定性的自然结果。非线性地，冷（$\sim 10^4$ K）气体的细丝应该具有与冷介质$\sim 10^{-4}$ pc！然而，观测表明，星系团中的原子丝要长得多，为$\sim 10$ kpc。宇宙线压力支持（或小尺度湍流磁压）可以解决这个矛盾：即使是扩散ICM中的小宇宙线压力，热压力的$\sim 10^{-4}$，也可以被压缩以在冷灯丝中提供显着的压力支持。这与用来解释在细丝中观察到的原子和分子线比的大量宇宙射线是定性一致的。
英文摘要：
Observations of the cores of nearby galaxy clusters show H$α$ and molecular emission line filaments. We argue that these are the result of {\em local} thermal instability in a {\em globally} stable galaxy cluster core. We present local, high resolution, two-dimensional magnetohydrodynamic simulations of thermal instability for conditions appropriate to the intracluster medium (ICM); the simulations include thermal conduction along magnetic field lines and adiabatic cosmic rays. Thermal conduction suppresses thermal instability along magnetic field lines on scales smaller than the Field length ($\gtrsim$10 kpc for the hot, diffuse ICM). We show that the Field length in the cold medium must be resolved both along and perpendicular to the magnetic field in order to obtain numerically converged results. Because of negligible conduction perpendicular to the magnetic field, thermal instability leads to fine scale structure in the perpendicular direction. Filaments of cold gas along magnetic field lines are thus a natural consequence of thermal instability with anisotropic thermal conduction. Nonlinearly, filaments of cold ($\sim 10^4$ K) gas should have lengths (along the magnetic field) comparable to the Field length in the cold medium $\sim 10^{-4}$ pc! Observations show, however, that the atomic filaments in clusters are far more extended, $\sim 10$ kpc. Cosmic ray pressure support (or a small scale turbulent magnetic pressure) may resolve this discrepancy: even a small cosmic ray pressure in the diffuse ICM, $\sim 10^{-4}$ of the thermal pressure, can be adiabatically compressed to provide significant pressure support in cold filaments. This is qualitatively consistent with the large population of cosmic rays invoked to explain the atomic and molecular line ratios observed in filaments.
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