压力各向异性驱动的动理学不稳定性对星系团中磁场放大的影响
The effect of pressure-anisotropy-driven kinetic instabilities on magnetic field amplification in galaxy clusters
AI总结：
本研究考虑压力各向异性驱动的动理学不稳定性效应，结合Modified GALFORM并合树与湍流小尺度发电机模型，发现星系团磁场放大需在z≥1.5开始才能在z=0达均分，为未来射电观测约束发电机阶段提供工具。
摘要：
星系团内介质（ICM）是星系团中低密度的弥散磁化等离子体，其位力温度高达10^8 K。在这些条件下，等离子体是弱碰撞性的，因此相对于局地磁场方向具有各向异性的压力张量。这会触发非常快速的、拉莫尔尺度的、由压力各向异性驱动的动理学不稳定性，从而改变磁场放大过程。我们研究了在典型大质量星系团演化过程中，通过湍流小尺度发电机实现的磁场放大，其中包含了动理学不稳定性的效应。本工作的一个具体目标是确定一个红移极限，发电机必须从该红移处开始放大磁场，才能在红移z=0时达到与湍流速度场的均分状态。我们为采用Modified GALFORM算法生成的并合树实现了各类等离子体物理量的一维径向剖面。我们假设湍流由暗物质晕的持续并合驱动，并针对三种不同磁化状态构建了雷诺数Re_eff对磁场的依赖关系的有效模型，其中包含了动理学不稳定性的效应。我们针对不同的Re_eff模型计算了磁场增长率。该模型得出，在更高红移处磁场增长率更高。对于所有考虑的情形，要在z=0时达到均分状态，磁场放大必须在起始红移z_start=1.5及更早开始。在湍流驱动尺度系统性更小的情形中，以及对于最高Re_eff的模型，达到均分的时间可以显著缩短。并合树是研究弱碰撞等离子体中磁场演化的有用工具。它们还可用于约束发电机的不同阶段，这些阶段有望被未来的射电望远镜观测到。
英文摘要：
The intracluster medium (ICM) is the low-density diffuse magnetized plasma in galaxy clusters, which reaches virial temperatures of up to 10^8 K. Under these conditions, the plasma is weakly collisional and therefore has an anisotropic pressure tensor with respect to the local direction of the magnetic field. This triggers very fast, Larmor-scale, pressure-anisotropy-driven kinetic instabilities that alter magnetic field amplification. We study magnetic field amplification through a turbulent small-scale dynamo, including the effects of the kinetic instabilities, during the evolution of a typical massive galaxy cluster. A specific aim of this work is to establish a redshift limit from which a dynamo has to start to amplify the magnetic field up to equipartition with the turbulent velocity field at redshift z=0. We implemented 1D radial profiles for various plasma quantities for merger trees generated with the Modified GALFORM algorithm. We assume that turbulence is driven by successive mergers of dark matter halos and construct effective models for the Reynolds number Re_eff dependence on the magnetic field in three different magnetization regimes, including the effects of kinetic instabilities. The magnetic field growth rate is calculated for the different Re_eff models. The model results in a higher magnetic field growth rate at higher redshift. For all scenarios considered, to reach equipartition at z=0, the amplification of the magnetic field has to start at redshift z_start=1.5 and above. The time to reach equipartition can be significantly shorter, in cases with systematically smaller turbulent forcing scales, and for the highest Re_eff models. Merger trees are useful tools for studying the evolution of magnetic fields in weakly collisional plasmas. They could also be used to constrain the different stages of the dynamo that could be observed by future radio telescopes.
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