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

这个项目的重点是星系团，已知最大的天体物理对象。这些是宇宙中物质凝聚在一起的基本单位。我们通过热的电离等离子体发射的x射线观察到这些结构，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阱中）。星系团似乎是自我调节的（热）动力系统：星系团内的气体缓慢地吸积到一个中心黑洞上——一个活动星系核，或AGN。但吸积过程并不干净，也不平静。吸积过程中产生的各种不同形式的辐射搅动并重新加热星系团气体。精确地说，这种加热是如何发生的，并使星团保持极高的x射线发射温度，一直是理论天体物理学中一个长期存在的问题。事实上，所谓的“加热问题”也延伸到许多其他天体物理环境中的热稀释等离子体，甚至导致人们猜测，新的基本粒子可能通过它们的衰变而参与其中。在这个项目中，我们采取更保守的策略，尽可能严格地研究稀释等离子体的动力学，跟踪从AGN到微观尺度上耗散的大规模破坏中注入的能量。在失去x射线卫星瞳号之前，该仪器对英仙座星系团进行了研究，首次直接测量了星系团内的湍流速度（通过发射线宽度）。从我们的观点来看，关键的结果是测量的值精确地符合湍流加热的正确数量级，以保持星系团气体免受热轫致冷却的辐射损失。这给了我们一定程度的信心，将气体的加热归因于波状运动（在这种情况下，浮力内部重力波和磁性阿尔芬波）和机械驱动的湍流的耗散。因此，在星系团气体加热是动态的这一概念的推动下，我们将研究星系团气体中的波系综如何传播，当然还有消散。气体在x射线团簇中是如此的稀薄，以至于在磁力线周围旋转的离子和电子在相互碰撞之前会形成很多这样的电路。波如何在这些条件下传播，以及宏观运动如何与由此产生的丰富的微观波动相互作用，就像基础等离子体物理学中的一个项目一样，还远未完全理解。我们将研究稀释磁化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受到某种形式的阻尼的热化，作为天体物理学中一个突出问题的可能解决方案。我们将使用解析技术，主要以线性化波色散计算的形式，以及使用众所周知的PLUTO代码进行数值模拟。自本世纪末以来，以稀等离子体新不稳定性形式出现的一系列新物理现象已经被发现，而这些波对星系团冷却流的影响和行为尚未被详细探索。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. Reynolds教授（剑桥）的初步努力的基础上，将PLUTO代码应用于稀释磁化等离子体中驱动力的一系列受控问题，以计算大规模机械干扰在最小尺度上作为热量消散的效率。这些结果最终将用于回答来自中心AGN的机械搅拌是否负责维持星系团x射线气体的热稳定性的问题。

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.