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

星系团的动力学、热力学和等离子体物理学：波阻尼和湍流加热

• 批准号: 2112095
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2112095
• 关键词: Dynamics; thermodynamics; plasma; physics; galaxy

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.

该项目的重点是星系团，这是已知最大的天体。这些是宇宙中物质凝聚在一起的基本单位。我们通过来自热电离等离子体的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。（这些重子被引力限制在与更大质量的暗物质成分相关的势阱中）。星系团似乎是一个自我调节的（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。但是，这种增长并不干净或安静。来自吸积过程的辐射，以各种不同的形式，搅动和重新加热团簇气体。这种加热是如何发生的，并使星系团保持非常高的X射线发射温度，这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释等离子体的动力学，跟踪从AGN注入到微观尺度上的大尺度破坏中的能量。在X射线卫星Hitomi丢失之前，该仪器对英仙座星团进行了研究，首次直接测量（通过发射线宽度）星系团内的湍流速度。从我们的观点来看，关键的结果是，测量的值是精确的正确的数量级的湍流加热，以保持集群气体对热韧致辐射冷却的辐射损失。这使我们对将气体加热归因于波动运动（在这种情况下，浮力内部重力波和磁阿尔芬波）和机械驱动湍流的耗散的想法有一定程度的信心。因此，受到团簇气体的加热是动态的这一概念的启发，我们将研究团簇气体中的波系综是如何传播的，当然，也是如何耗散的。在X射线团中，气体是如此的稀薄，以至于围绕磁力线螺旋运动的离子和电子在相互碰撞之前会形成很多这样的回路。波在这些条件下如何传播，以及宏观运动如何与丰富的微观波动动物园相互作用，这些波动从它们中得到反馈，就像基础等离子体物理学中的一个项目一样，还远远没有完全理解。我们将研究波在稀磁化等离子体中的数学行为，这既是为了它本身，也是因为波不可避免地被某种形式的阻尼热化，作为天体物理学中一个突出问题的可能解决方案。我们将利用分析技术，主要是线性波色散计算的形式，以及使用著名代码PLUTO进行数值模拟。自上个世纪以来，人们发现了大量以稀等离子体中新的不稳定性为形式的新物理，而这些波对团簇冷却流的影响和行为还有待于详细探索。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教授的初步努力的基础上再接再厉。Reynolds（剑桥）使用PLUTO代码对一系列在稀磁化等离子体中驱动强迫的控制问题进行计算，以计算大尺度机械破坏在最小尺度上作为热耗散的效率。这些结果将最终被用来回答这样一个问题，即来自活动星系核中心的机械搅动是否是维持星系团X射线气体热稳定性的原因。