Dynamics, thermodynamics and plasma physics of galaxy clusters: wave damping and turbulent heating

这个项目的重点是星系团，这是已知的最大的天体物理物体。这些是物质在宇宙中凝聚在一起的基本单位。我们通过热的、电离的等离子体的X射线发射来观察这些结构，这些等离子体构成了它们的大部分可见质量。(这些重子在引力作用下被限制在与质量大得多的暗物质成分相关的势井内)。星系团似乎是自我调节的(热)动力系统：星系团内的气体慢慢地附着到中央黑洞-活动星系核，或称AGN。但这种吸积并不干净，也不安静。吸积过程中排放的气体会以各种不同的形式搅动和重新加热星系团气体。确切地说，这种加热是如何发生的，并允许星系团保持非常高的X射线发射温度，这一直是理论天体物理学中的一个长期问题。事实上，所谓的“加热问题”也延伸到了许多其他天体物理环境中的热稀薄等离子体，甚至引发了人们的猜测，即新的基本粒子可能通过衰变而参与其中。在这个项目中，我们采取了更保守的策略，尽可能严格地研究稀薄等离子体的动力学，跟踪从活动星系核到微观尺度的大范围分裂中注入的能量。在X-射线卫星Hitomi丢失之前，该仪器对英仙座星系团进行了研究，首次(通过发射线宽度)直接测量了星系团内的湍流速度。从我们的观点来看，关键的结果是，测量的数值精确地处于湍流加热的正确数量级，以保持星系团气体免受来自热韧致辐射冷却的辐射损失。这给了我们一定程度的信心，将气体的加热归因于波动运动(在这种情况下，是浮力内部重力波和磁性阿尔芬波)和机械驱动的湍流的消散。因此，在团簇气体加热是动态的这一概念的推动下，我们将研究团簇气体中的波系综如何传播，当然，也是如何消散的。这种气体在X射线星团中是如此稀薄，以至于在相互碰撞之前，围绕磁力线盘旋的离子和电子形成了非常多的这样的电路。波如何在这些条件下传播，以及宏观运动如何与丰富的微观波动动物园相互作用，这些都是人们远未完全了解的，就像基础等离子体物理学中的一个项目一样。我们将研究稀磁等离子体中波的数学行为，这既是为了它本身的缘故，也是因为波不可避免地被某种形式的阻尼热化，作为天体物理中一个突出问题的可能解决方案。我们将利用分析技术，主要是线性化波频散计算的形式，以及使用著名的程序Pluto进行的数值模拟。自本世纪末以来，以稀薄等离子体中新的不稳定性的形式发现了许多新的物理，而这些波对星团冷却流动的影响和行为还没有详细地探讨。非线性物理，特别是能量级联和耗散，需要更大的数值模拟。我们将在与C·雷诺兹教授(剑桥)的初步工作的基础上，使用冥王星代码来解决稀磁等离子体中驱动作用力的一系列受控问题，以计算大规模机械破坏在最小尺度上作为热量的消散效率。这些结果最终将被用来回答来自中心活动星系核的机械搅动是否对维持星团X射线气体的热稳定性负责。

This project focuses on galaxy clusters, the largest known astrophysical objects. These are the basic units in which matter coherently clumps together in the Universe. We observe these structures via X-ray emission from the hot, ionised plasma that makes up most of their visible mass. (These baryons are gravitationally confined within the potential well associated with the much more massive dark-matter component). Clusters appear to be self-regulated (thermo)dynamical systems: the intracluster gas slowly accretes onto a central black hole --- an active galactic nucleus, or AGN. But the accretion is not clean or quiet. Emission from the accretion process, which take a variety of different forms, stirs and reheats the cluster gas. Precisely how this heating occurs and allows the clusters to maintain their very high X-ray emission temperatures, has been a longstanding problem in theoretical astrophysics. In fact, the so-called "heating problem" extends to hot dilute plasmas in many other astrophysical environments as well, even leading to speculation that new fundamental particles could be involved via their decay. In this project, we take the more conservative tack of investigating the dynamics of dilute plasmas as rigorously as possible, following the energy injected in the large scale disruptions from the AGN down to dissipation at the microscopic scales.Before the loss of the X-ray satellite Hitomi, the instrument carried out a study of the Perseus Cluster, directly measuring for the first time (via emission line widths) the turbulent velocities within a galaxy cluster. The key result from our point of view was that the values measured were precisely of the right order of magnitude for the turbulent heating to maintain the cluster gas against radiative losses from thermal Bremsstrahlung cooling. This gives us some degree of confidence in ideas attributing the heating of the gas to the dissipation of both wavelike motions (in this case buoyant internal gravity waves and magnetic Alfven waves) and mechanically driven turbulence. Thus motivated by the notion that the heating of the cluster gas is dynamical, we shall study how ensembles of waves in cluster gas propagate and, of course, dissipate. The gas is so extremely dilute in X-ray clusters, that ions and electrons spiralling around magnetic lines of force make very many such circuits before colliding with each other. How waves propagate under these conditions and how macroscopic motions interact with rich microscopic zoo of fluctuations that feeds off them is far from fully understood, just as a project in fundamental plasma physics. We will study the mathematical behaviour of waves in a dilute magnetised plasma both for its own sake and, because the waves are inevitably thermalised by some form damping, as a possible solution to an outstanding problem in astrophysics.We will make use of analytic techniques, primarily in the form of linearised wave dispersion calculations, and numerical simulations, using the well-known code PLUTO. Since the turn off the century, a host of new physics in the form of novel instabilities in dilute plasmas has been discovered, and the consequences and behaviour of these waves for cluster cooling flows have yet to explored in detail. The nonlinear physics, in particular the energy cascade and dissipation, requires larger numerical simulations. We will build on our preliminary efforts with Prof C. Reynolds (Cambridge) to use the PLUTO code on a series of controlled problems of driven forcing in a dilute magnetised plasmas to calculate how efficiently the large scale mechanical disruptions are dissipated as heat at the smallest scales. These results will then ultimately be used to answer the question of whether mechanical agitation from a central AGN is responsible for maintaining the thermal stability of cluster X-ray gas.