Most of my research lately has been directed at understanding clusters of galaxies and how their growth and development reflects the evolution of the universe as a whole.  Galaxy clusters are excellent tracers of structure formation in the universe because they are the largest structures that have so far achieved dynamical equilibrium.  They are also excellent laboratories for studying the processes that regulate galaxy evolution because they are among the few places where we can observe the hot gas that surrounds those galaxies.  Some of the topics I find particularly interesting are briefly described below.  If you want more details, see my most recent scientific papers.

Galaxy Clusters and Cosmology

Almost all of the yellow galaxies in the photo at left belong to the same galaxy cluster. Like most clusters, this one comprises about 83% dark matter, 15% hot gas, and only 2% stars and is therefore held together primarily by the gravity of dark matter.  We’re trying to understand what the growth rate of galaxy clusters is telling us about how the gravity of dark matter creates structure in the universe and how repulsive forces attributed to dark energy are bringing the era of structure formation to a close.

Galaxy Clusters and Supermassive Black Holes

Why hasn’t more of the hot gas in galaxy clusters turned into stars?  The answer apparently has something to do with the supermassive black holes at the centers of many galaxy clusters.  In the composite picture at right, red shows X-ray emission from a cluster’s hot gas and blue shows radio-wave emission from high-speed electrons flowing from the vicinity of the central black hole and interacting with the hot gas.  We’re trying to understand how interactions like this prevent the hot gas from cooling and forming more stars.

The Universe’s Largest Galaxies

The centers of galaxy clusters harbor the largest galaxies in the universe.  Some of these giant galaxies are quite unusual, with extensive nebulae (red in photo at left) and active star formation.  Giant galaxies with these properties are seen only in clusters in which the central atmosphere of hot gas is particularly dense, and we want to know why.  The answer may hinge on the interplay between heat conduction and radiative cooling in the cores clusters, so our group is now performing numerical simulations of these processes.

Intergalactic Star Formation

Ming Sun discovered the fascinating galaxy at right while working in our group at Michigan State.  As this galaxy moves through the hot gas of its galaxy cluster, the interaction is stripping its gas clouds while triggering a burst of intergalactic star formation within the stripped clouds.  Blue streaks show X-ray emission from stripped gas.  Red blobs are nebulae marking young clusters of newborn stars.  We’d like to know how common this process is, and we’re wondering why there are two tails of stripped gas.

Mark’s Research