Dr McKay and his group will study a set of rich clusters of galaxies, the 'MaxBCG catalog', which they have identified from the galaxy database of the Sloan Digital Sky Survey (SDSS). They will reprocess the images from the SDSS to obtain better measures of the galaxy brightness. They will use the Magellan telescopes to measure galaxy motions within the cluster, to estimate the mass of each cluster. Measuring weak gravitational lensing -- the way in which the cluster's gravitational pull distorts the path of light from distant galaxies lying behind it -- will provide additional mass estimates. The results of this study will relate measures of the optical richness (the number and brightness of its member galaxies) of each cluster to its mass.
The results of this work will be used to test theories of cosmic structure formation, which can predict how the mass that galaxy clusters should build up through cosmic time. Galaxy clusters are widely used in probing dark energy and in cosmological tests of General Relativity, so this research will have a broad scientific impact. Dr. McKay and his group have an excellent record in mentoring undergraduate researchers, and will involve both undergraduate and graduate students in this work. He will also teach a research seminar for incoming undergraduates in which they will perform analysis projects based on public SDSS data.
Over the last 20 years, we have learned that our universe was born 13.7 billion years ago, that it has been expanding ever since, and that within this expansion gravity has brought matter together, first into stars, then galaxies, and finally into clusters of galaxies. Galaxy clusters are the largest fully formed objects in the Universe. Just as forests are made of trees and societies of individuals, galaxy clusters are made of galaxies. As the final stage of structure formation, the number and nature of galaxy clusters we discover is very sensitive to the balance cosmic expansion and the implacable pull of gravity. Counting clusters and measuring their properties with care can teach us much about how expansion and gravity are related, ultimately providing insight into the two great cosmic mysteries of our time: dark matter and dark energy. Indeed observations of clusters provided the first evidence that structures in the universe were dominated by invisible dark matter, and they have played a key role in confirming that dark energy is driving the expansion of the universe to accelerate. The team assembled for this project has studied galaxy clusters for more than a decade. Before this project began, we used data from the Sloan Digital Sky Survey to identify tens of thousands of galaxy clusters, creating a kind of census which allows us to begin exploring them in more detail. In this project, we have sought to understand how to use observations of clusters to better explore dark matter and dark energy. In our main line of research, we have examined how best to determine important properties of individual clusters like their mass and the best way to locate their centers and sizes. Estimating cluster masses is tricky. Most of a cluster’s mass is dark matter, inaccessible to direct observation. Fortunately, clusters are still easy to see: they are made of hundreds of galaxies, surrounded by ionized gas so hot that it emits x-rays and distorts the spectrum of background light heading toward us. They have another, more subtle effect as well, distorting the images of galaxies which lie behind them. A key part of this project has involved comparing our observations of the number and types of galaxies within clusters to observations of the x-rays they emit and the extent to which they ‘lens’ the images of background galaxies. Combining these various observations in a careful way gives us a chance to understand them all. As part of this project, we have created the best and most reliable ways to use observations of the galaxies in clusters to determine their masses. This work is especially important now, as several large new projects which will detect still more galaxy clusters are just beginning, including both the Dark Energy Survey and the Large Synoptic Survey Telescope. These future projects will use what we have learned to great advantage. In addition to identifying cluster masses, it is important to accurately and automatically identify their centers. To pursue this problem, we have brought to galaxy cluster research a set of tools based on ideas derived from network theory, a generally separate field of physics and social science research. This new vision, seeing the galaxies in a cluster as an interacting network, works well for identifying cluster centers, as well as holding promise for more complex characterization of the galaxy distribution in the future. Galaxy clusters are thought to be embedded in the large scale structure of the universe in a very particular way, lying at the intersections of long, rather narrow ‘filaments’ of galaxies. These filaments are devilishly hard to detect individually, indeed the first really reliable detection of a single filament was accomplished as part of this project. We have created a new method for their study, using the fact that pairs of galaxy clusters should be connected by filaments. Taking many pairs of clusters, we can study the average filaments between them in great detail. We are just beginning to use this promising new method, and look forward to applying it in the upcoming new surveys. This project has had a number of impacts outside the professional astrophysics community, especially in the training of a large number of young scientists. We are especially proud to have worked with so many undergraduate researchers, including a number of female and underrepresented minority scientists. While not all of these students continue on in physics and astronomy, they carry what they have learned about research excellence with them into medical school and education careers, where they are still sorely needed.
