Co-Evolution of Cosmic Rays and Thermal Plasma in Galaxy Clusters

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This project will explore the physics and observational consequences of cosmic rays in galaxy clusters. Clusters of galaxies formed early in the history of the Universe, and are still growing by pulling in matter from their outskirts and occasionally merging with other clusters. As matter is added, it creates heat, much as meteors are heated as they fall through the atmosphere of the Earth. At the same time, the environments surrounding supermassive black holes at the centers of galaxy clusters spew forth matter and energy in spectacular bursts. The medium into which all this, heat, matter, and energy are poured is a plasma – a fully ionized gas so hot that instead of emitting visible light, it glows in x-rays. About one in ten billion of the charged particles within this plasma is a cosmic ray - so energetic that it travels at nearly the speed of light. The goal of this project is to unravel this complex dance of mass and energy input, understand how it creates a plasma and cosmic ray mixture, and predict how it will appear under the lens of radio, x-ray, and gamma-ray telescopes existing now and planned for the future. The project will also provide research training for graduate and undergraduate students from traditionally under-represented groups. X-ray observations of galaxy clusters reveal that about 40% of their baryonic matter is in the form of a pervasive, 10 – 100 keV plasma known as the Intracluster Medium (ICM). The ICM carries a magnetic field with tangled topology and hosts an energetically significant population of relativistic cosmic rays, which are observed at radio frequencies and were expected to be detected in gamma-rays of hadronic origin, which puzzlingly have never been seen. Understanding energy partitioning and transport in the ICM, from global scale gravitational infall at the cluster outskirts and supermassive black hole activity at the center, to the microscale waves and instabilities that thrive in the absence of frequent Coulomb collisions, is a multi-scale non-equilibrium plasma physics problem. Progress has only recently become possible due to advanced computing resources and algorithmic breakthroughs that enable numerical simulation of ICM plasma almost from first principles. A team of graduate and undergraduate students led by faculty members at the University of Wisconsin - Madison collaborating with scientists at the Los Alamos National Laboratory will use these opportunities to comprehensively model, for the first time, the joint evolution of thermal plasma, relativistic cosmic rays, and turbulence. Radiative outputs of the calculations are expected to lead to detailed predictions that will be tested by radio, x-ray, and gamma-ray observations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.