I study galaxy clusters and their outskirts. Galaxy clusters are the largest collapsed objects in the universe, and thus provide a powerful probe of the growth and evolution of structure in the universe. Clusters are still forming today, and many galaxies in their outskirts are falling into the cluster for the first or second time. Velocity surveys of the outskirts of these clusters measure the mass on large scales and show the state of cluster assembly. Many of these galaxies are aligned along massive filaments like those seen in numerical simulations. Galaxy properties change dramatically from cluster centers to their edges, although the physical mechanisms responsible for this change are still hotly debated. Are cluster galaxies formed differently from field (non-cluster) galaxies? Or do cluster galaxies resemble field galaxies and then evolve in harsh cluster and group environments? If the latter, does this evolution occur only in massive clusters or does it occur in the less massive groups which merge to form the cluster?

I am also using clusters to try to understand the properties of dark energy. Alexey Vikhlinin at the CfA is leading a team getting Chandra observations of 41 X-ray selected clusters at moderate redshift. The Chandra data measure the masses of the clusters. Once you know the masses of the clusters, you can reconstruct the history of structure formation, which depends sensitively on the amount of and properties of dark energy. I am following up these clusters at multiple wavelengths. We were awarded Spitzer time to observe star formation and its evolution in these clusters. I am using the WIYN telescope to obtain multicolor imaging to identify likely cluster members. We have recently been awarded time on the Kitt Peak 4-meter telescope to observe these clusters at near-infrared wavelengths. These images will measure the stellar masses of the cluster galaxies. All of the above data can be combined to measure scaling relations between cluster masses and simple observables like those expected in future large cluster surveys. We expect that this sample will be the best studied sample of "typical" clusters at cosmologically important redshifts.

BA, Rice University (1997)
AM, PhD, Harvard University (2003)