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,
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.
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.
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