Galaxy clusters
Introduction

The Bullet Cluster 1E0657 (NASA/CXC)
At the University of Illinois, graduate students involved in this work include Hsiang-Yi (Karen) Yang (now at the University of Michigan) and Paul Sutter (now at the Institute for Astrophysics in Paris), and postdocs include Alessandro Gardini (now at the University of Oslo).
Galaxy cluster mergers
Early in the history of the Universe, baryons ("ordinary" matter made of protons and neutrons) and dark matter were distributed almost uniformly throughout space. However, in some places the density of matter was slightly greater than average, and in some places it was slightly less. As the Universe expanded, gravity tended to pull together the matter in the denser regions, evacuating the less-dense regions. This process, called gravitational instability, was responsible for the formation of galaxies and clusters of galaxies.
Current thinking, backed by considerable evidence from the cosmic microwave background (CMB) radiation and the large-scale distribution of galaxies, holds that dark matter is "cold," ie. dark matter particles move at much less than the speed of light. Under these conditions gravitational instability tends to build large objects through mergers of smaller ones. We see many examples of merging galaxies and clusters of galaxies, such as the Bullet Cluster shown above. Because of the enormous lengths of time these mergers take (billions of years for clusters), we essentially see only "snapshots" of these mergers at different stages and must piece together how they work from these snapshots and our understanding of the laws of physics.

FLASH galaxy cluster merger simulation
(J. ZuHone, B. Gallagher)
Feedback from active galactic nuclei
Many, perhaps all, galaxies host supermassive (more than 106 solar masses) black holes at their centers. When gas falls into a black hole, it generally does not fall straight in, because it has orbital angular momentum that must be dissipated. Instead it spirals into the hole in the form of an accretion disk, transferring its angular momentum to gas that is farther out through the action of viscosity (thought to be generated by magnetic fields and turbulence). Even so, not all of the gas makes it into the black hole; some of it is funnelled by magnetic fields into a pair of powerful jets that carry the gas away from the black hole at speeds close to the speed of light. Galaxies that host such an actively "feeding" black hole are said to harbor an active galactic nucleus or AGN. AGN are regarded as the root cause of a variety of interesting phenomena, such as quasars and radio lobes.
One major puzzle about galaxy clusters is why many of them have central gas densities and temperatures such that the gas should radiate away its internal "heat" over a time shorter than the age of the Universe, but nevertheless these clusters show no evidence of such cooling. The answer probably is that some form of heat input must be counterbalancing the radiative cooling, but what is this source of heat? Its rate must be tuned to the cooling rate, or else it would overheat the cluster. It must also heat in a distributed way that does not upset the stability of these clusters. One possible answer, which my group and others are investigating, is provided by AGN.

AGN bubbles in Perseus cluster
(NASA/CXC)
Computer simulations can help us to understand how this might work. However, a major problem is that the black hole accretion disk in an AGN is a billion times smaller than the spatial resolution we can achieve today in galaxy cluster simulations. Thus, rather than directly solving the equations that describe the motion of matter in the accretion disk, we must come up with a set of ad-hoc equations that model the feedback loop and use them inside a direct simulation of the cluster environment. Our group is comparing a number of these candidate models to determine which ones best fit observations. The ultimate goal is to use the most successful models in large cosmological simulations so that we can study the impact of AGN feedback on the cluster population as a whole.
Magnetic fields in galaxy clusters
Galaxy clusters are pervaded by magnetic fields with strengths on the order of a microgauss (about 1% of the Earth's magnetic field). Because the intracluster medium is very hot, it is almost completely ionized, and therefore it behaves as a plasma that can interact with this field. The intracluster magnetic field is not strong enough to push around the plasma in dramatic ways like the Sun's magnetic field can, but it does affect the way the plasma conducts heat, and it plays an important role in accelerating electrons and protons at shock fronts. Our group and others are trying to understand the origin of the intracluster magnetic field by performing magnetohydrodynamic (MHD) simulations of cluster formation that include different candidate sources of magnetic field, such as magnetized AGN jets.