Cooling Flows and Thermal Conduction in Galaxy-Cluster Plasmas

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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).***