Plasma Physics in Clusters of Galaxies

Clusters of galaxies are the largest gravitationally bound objects in the universe. They have been identified on the sky with optical telescopes, with which rich groups of galaxies can be seen (as in the photo on the left). This is not the whole story, however; X-ray telescopes show us a very different side of clusters, revealing that they have hot gaseous atmospheres (as in the photo on the right, which is on approximately the same spatial scale). This gas is known to be ionized and magnetized: thus it is more appropriately called a plasma.

Some clusters have another component: relativistic electrons. These particles can be seen by radio telescopes such as the VLA due to the synchrotron emission (called a "radio halo") they produce as they spiral in the cluster's magnetic field. These very energetic particles permeate the cluster atmosphere, much as cosmic rays permeate the interstellar gas within our own galaxy.

These electrons share the same mystery as do our local cosmic rays. We do not understand how they are accelerated to such high energies, how they can stay at such high energies, or how they propagate within the cluster atmosphere. The answer is probably connected to the recent discovery that clusters are dynamic, ever changing structures. Although they have been around since quite early in the history of the universe, they are still evolving today as new clumps of matter fall into the cluster's potential well. This growth must drive turbulence in the plasma atmosphere. We believe that such turbulence is responsible for maintaining these relativistic electrons at high enough energies that radio telescopes can see them.

Our group is trying to understand how these "cosmic rays in clusters" come to be. Our work combines observations and theory. We are carrying out a VLA study of a complete sample of rich clusters, in order to determine how common radio haloes are. Are they rare (and thus due to a special event in the life a cluster), or common (and thus due to "cluster weather")? We are also studying a particular type of plasma turbulence -- lower hybrid turbulence -- which is known -- from laboratory and spaace work. We suspect this type of turbulence may also be generated in clusters of galaxies. We are developing theoretical models to determine how important this process is in the cluster environment, and to compare it to other mechanisms which might produce relativistic particles.

--- Jean Eilek; with T. Markovic (NMT), F. Owen (NRAO), J. Weatherall (FAA), G. Benford (UCI)