Researchers unravel the inner workings of heat conduction in galaxy clusters

University of Oxford Press Release

The inner workings of heat conduction in galaxy clusters have been unravelled by a collaboration of international researchers led by the University of Oxford, University of Rochester and the University of Chicago.

Most matter in galaxy clusters is in the form of tenuous ionised gas called plasma that is threaded by magnetic fields and is in a turbulent state; in observing many of these clusters of galaxies, astronomers have been facing a difficult conundrum: they all appear much hotter than expected.

Paper author Dr Jena Meinecke and the research team used the largest laser system in the world – the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California – to create a replica of the plasma conditions expected to occur in the clusters of galaxies.

Co-author Professor Alexander Schekochihin, Department of Physics at the University of Oxford said:

‘How energy is injected into the plasma that fills galaxy clusters by the violently active galaxies at their centres, how it is then spread around and heats up the entire enormous system, producing the X-ray glow that observatories like Chandra X-ray Observatory pick up – these are fundamental questions about the largest building blocks of our Universe. Both observations and the logic of our theoretical models suggest that heat conduction in these plasmas is strongly suppressed compared to naïve expectations. Several schemes for such suppression have been theorised about and simulated numerically, but very tentatively.

‘Here suddenly we have it in a real laboratory plasma – and so experiment now has a chance to leapfrog theory in helping sort out the basic properties of an astrophysical plasma, an exciting prospect.’

Dr Meinecke, who was at the University of Oxford during the research and is now at Boise State University, continued:

‘The experiments conducted at the NIF are literally out of this world.  Capable of bringing the powerful dynamics of the Universe to the laboratory, the NIF truly provides opportunities to step into the unknown.’

The researchers used laser beams to vaporise plastic foils and generate a turbulent and magnetised plasma in the experiments at NIF.

Dr Archie Bott, Department of Astrophysical Sciences at Princeton University said:

‘What is unique in these NIF experiments is that electrons in the plasma collide sufficiently infrequently with each other that they end up following the tangled magnetic field lines.

‘This phenomenon, which is precisely what is believed to occur in clusters of galaxies, gives rise to suppressed heat conduction,’ said Dr Bott

This effect is clearly seen in the laboratory data: the measurements show pockets of hot plasma that persist in time and heat cannot escape.

The principal investigator for the experiments Professor Gianluca Gregori, Department of Physics at the University of Oxford said:

‘This work is an important stepping stone to understanding the microscopic processes that occur in plasmas that are both magnetised and turbulent. The experimental findings are somewhat surprising as they demonstrate that energy is transported in ways that are very different from what we would have expected from simple theories.’

‘This is indeed an astonishing result,’ confirms Professor Petros Tzeferacos, director of the Flash Center of Computational Science, who led the simulation efforts to design and help interpret the NIF experimental campaign.

‘To model the NIF experiments, we brought to bear the full array of physics capabilities of FLASH, the multi-physics simulation code we develop. The FLASH simulations were key for untangling the physics at play in the turbulent, magnetised plasma, but the level of thermal transport suppression was beyond what we expected.’

Although the simulations reproduce the experimental results by controlling the electron heat transport, the microscopic mechanism ultimately responsible for the observed suppression remains unclear.

Preparations are underway for further work using the NIF laser to look at the details of these interactions.

‘These experiments provide insight into complex physics processes and also raise additional questions that we hope to answer in upcoming NIF Discovery Science experiments with an optimised target design and diagnostic configuration,’ said Dr James Steven Ross, the project’s liaison scientist at the LLNL.

These experiments demonstrate how laboratory explorations can help the understanding of astrophysical systems in a way that is complementary to observations. Co-author Professor Don Lamb from the University of Chicago said:

‘The Discovery Science Program at the National Ignition Facility – the most energetic laser in the world – made it possible for us to study fundamental physical processes that are essential to understanding the hot, tenuous, magnetised plasma in the cores of galaxy clusters.’

ENDS