BUOYANCY INSTABILITIES IN GALAXY CLUSTERS: CONVECTION DUE TO ADIABATIC COSMIC RAYS AND ANISOTROPIC THERMAL CONDUCTION
BUOYANCY INSTABILITIES IN GALAXY CLUSTERS: CONVECTION DUE TO ADIABATIC COSMIC RAYS AND ANISOTROPIC THERMAL CONDUCTION
复制标题
星系团中的浮力不稳定性：绝热宇宙射线和各向异性热传导引起的对流
DOI：
10.1088/0004-637x/699/1/348
复制
发表时间：
2009
期刊：
影响因子：
--
通讯作者：
I. Parrish
中科院分区：
文献类型：
--
作者：
P. Sharma;B. Chandran;E. Quataert;I. Parrish
利用线性稳定性分析和二维和三维非线性模拟，我们研究了热等离子体和相对论性（宇宙射线）等离子体相结合的浮力不稳定性的物理原理，其动机是应用于星系团。我们认为，与大长度尺度上的浮力时间相比，宇宙射线的扩散时间可能很长，因此宇宙射线实际上是绝热的。如果宇宙射线压力 pcr ≳热压力的 25%，并且宇宙射线“熵”pcr/ρ4/3（其中 ρ 是热等离子体密度）向外减小，则宇宙射线驱动绝热对流不稳定性，类似于恒星中的史瓦西对流。星系团核心的全局模拟表明，这种不稳定性通过降低宇宙射线熵梯度并驱动有效的对流和湍流混合而饱和。在宇宙射线压力可以忽略不计的簇核半径较大的情况下，热等离子体对于热通量驱动的浮力不稳定性（HBI）是不稳定的，HBI是由各向异性热传导和背景传导热通量产生的对流不稳定性。 HBI 通过重新排列磁场线使其基本上垂直于局部引力场而饱和；由此产生的湍流也主要在垂直平面上混合等离子体。宇宙射线驱动的对流和 HBI 可能有助于重新分布 Ia 型超新星在星团中产生的金属。我们的计算表明，星系团的绝热模拟可以人为地抑制热等离子体的混合。当包括各向异性热传导时，热等离子体的浮力响应不受稳定熵梯度的控制，并且混合（由合并、宇宙射线浮力等驱动）更有效。这种混合可能有助于宇宙射线分布在整个星团体积中。
Using a linear stability analysis and two- and three-dimensional nonlinear simulations, we study the physics of buoyancy instabilities in a combined thermal and relativistic (cosmic ray) plasma, motivated by the application to clusters of galaxies. We argue that the cosmic-ray diffusion time is likely to be long compared to the buoyancy time on large length scales, so that cosmic rays are effectively adiabatic. If the cosmic-ray pressure pcr is ≳25% of the thermal pressure, and the cosmic-ray “entropy” pcr/ρ4/3 (where ρ is the thermal-plasma density) decreases outward, cosmic rays drive an adiabatic convective instability analogous to Schwarzschild convection in stars. Global simulations of galaxy cluster cores show that this instability saturates by reducing the cosmic-ray entropy gradient and driving efficient convection and turbulent mixing. At larger radii in cluster cores where cosmic-ray pressure is negligible, the thermal plasma is unstable to the heat-flux-driven buoyancy instability (HBI), a convective instability generated by anisotropic thermal conduction and a background conductive heat flux. The HBI saturates by rearranging the magnetic field lines to become largely perpendicular to the local gravitational field; the resulting turbulence also primarily mixes plasma in the perpendicular plane. Cosmic-ray-driven convection and the HBI may contribute to redistributing metals produced by Type Ia supernovae in clusters. Our calculations demonstrate that adiabatic simulations of galaxy clusters can artificially suppress the mixing of thermal plasma. When anisotropic thermal conduction is included, the buoyant response of the thermal plasma is not governed by the stable entropy gradient, and mixing (driven by mergers, cosmic ray buoyancy, etc.) is more effective. Such mixing may contribute to cosmic rays being distributed throughout the cluster volume.