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

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.