IllinoisPaul Ricker



In this section

Galaxy clusters


Bullet Cluster
The Bullet Cluster 1E0657 (NASA/CXC)
Clusters of galaxies are the largest gravitationally bound objects in the universe. Their formation and evolution are complex, large-scale events, involving many interacting physical processes, which span a significant fraction of the age of the universe. In addition to being intrinsically complex and interesting, their properties permit us to constrain many quantities of cosmological interest, such as the ratio of baryonic to dark matter and the rate of expansion of the universe. Despite the name "cluster of galaxies," the galaxies themselves contribute the least, only a few percent, to the total mass of a cluster. The remainder consists of diffuse, hot gas (the intracluster medium, or ICM) and an unseen, presumably collisionless component which is needed to explain the gravitational stability of clusters (the dark matter).

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.

Galaxy cluster merger
FLASH galaxy cluster merger simulation
(J. ZuHone, B. Gallagher)
Together with Donald Lamb and John ZuHone at the University of Chicago and Craig Sarazin, Scott Randall, and Daniel Wik at the University of Virginia, we have used large computer simulations to study the effects of head-on and offset mergers on the observable properties of clusters, the generation and mainenance of turbulence in the intracluster medium, scatter in the relationship between cluster mass and X-ray temperature, and the generation of rings of dark matter. We have also looked at the detectability of head-on mergers viewed along the line of sight. In general, mergers can have dramatic effects on the observed brightness and temperature of individual clusters, but when considered as members of the larger population of clusters, these merging clusters have surprisingly little effect on the overall mass-observable statistics of the population. Mergers play an important role in seeding and driving turbulence because the dark matter does not feel pressure forces and takes longer to settle down after a merger. As it relaxes, its gravitational interaction with the baryons continues to pump energy into the intracluster medium. These motions can take previously stable gas configurations and make them convectively unstable, promoting the development of turbulent eddies.

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
AGN bubbles in Perseus cluster
Most galaxy clusters have at their centers a single very massive elliptical galaxy known variously as a cD galaxy, brightest cluster galaxy (BCG), or giant elliptical galaxy. Often these cD galaxies have an AGN at their centers. The jets produced by these AGN are known to slow down and form enormous buoyant bubbles as they collide with the intracluster medium. One idea is that since the rate of heat deposition due to these bubbles depends on the strength of the AGN jets, and since the jet strength depends on the black hole accretion rate, and since the black hole accretion rate depends on the cooling of the surrounding intracluster gas, this scenario provides a nice feedback loop that could explain how these "cool core" clusters maintain their balance.

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.