Cooling Flows and Thermal Conduction in Galaxy-Cluster Plasmas

星系团是观测到的宇宙中最大的有组织结构之一，在我们理解整个宇宙的演化中起着重要的作用。有趣的是，这些星系团在星系之间的巨大空间中拥有大量的等离子体（热电离气体）。一般来说，星团等离子体的质量比星团内所有星系中所有恒星的质量都要大得多。由于等离子体是星团的重要组成部分，它的演化对天文学家来说是一个重要而具有挑战性的问题。随着x射线卫星的出现，天文学家发现星团内等离子体通过释放大量高能x射线来冷却。当星系团内的等离子体冷却时，等离子体内部的压力降低，这使得重力压缩等离子体并迫使其向内靠近星系团的中心。等离子体向内流动的速度目前在天体物理学界引起了相当大的争议。一些天文学家认为星团内等离子体的辐射冷却被加热所抵消。例如，当靠近星团中心的等离子体冷却时，热量可以从星团核心周围较热的等离子体向内传导。这样的加热维持了核心中等离子体的压力，降低了流入的速度。其他天文学家认为，星团等离子体的加热是最小的，因此大量的星团内等离子体被吸积到星团的中心。这个项目将通过确定星系团的热传导效率来帮助解决这一争议。微妙之处在于簇内等离子体是湍流和磁化的。从热区到冷区携带热量的粒子主要沿着磁场运动，因此，如果磁场具有纠缠的拓扑结构，热量的传输就会减慢。这个项目将使用紊流等离子体的数值模拟来确定这种纠缠磁场中的热传导率。本项目由美国国家科学基金会星系外天文学和宇宙学项目（AST/EXC）资助

AST 0098086ChandranGalaxy clusters are among the largest organized structures in the observed universe and play an important role in our understanding of the evolution of the universe as a whole. Intriguingly, these clusters possess vast quantities of plasma (hot ionized gas) in the enormous spaces between their galaxies. Typically, there is much more mass in a cluster's plasma than in all the stars in all the galaxies within a cluster. Because this plasma is such an important constituent of a cluster, its evolution poses an important and challenging problem for astronomers.With the advent of x-ray satellites, astronomers discovered that intracluster plasmas cool by emitting huge amounts of high energy x-rays. As the plasma within a cluster cools, the pressure within the plasma decreases, which enables gravity to compress the plasma and force it inwards towards the cluster's center. The rate at which plasma flows inwards is currently the subject of considerable controversy within the astrophysical community. Some astronomers believe that the radiative cooling of intracluster plasma is offset by heating. For example, as the plasma near a cluster's center cools, heat can be conducted inwards from the hotter plasma surrounding a cluster's core. Such heating sustains the pressure of the plasma in the core, reducing the rate of inflow. Other astronomers believe that heating of cluster plasma is minimal, so that large quantities of intracluster plasma are accreted towards a cluster's center. This project will help resolve this controversy by determining the efficiency of thermal conduction in galaxy clusters. The subtlety is that intracluster plasmas are turbulent and magnetized. The particles which carry heat from hot regions to cold regions are constrained to move primarily along the magnetic field, so that if the magnetic field has a tangled topology, the transport of heat is slowed. This project will use numerical simulations of turbulent plasmas to determine the rate of heat conduction amidst such tangled magnetic fields. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***