ACTIONS
1998INTERACTIONS
1998
The Carnegie Mellon Cluster Hunters
by Kathy Romer
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Kathy Romer received a bachelor's degree in physics and astrophysics at Manchester University, England, in 1990. She completed her thesis work on observations of the large scale structure of the universe at Edinburgh University, Scotland, graduating in 1995. Kathy was a postdoctoral researcher at Northwestern University before joining Carnegie Mellon as a postdoctoral researcher in 1997. |
For nearly a decade I have been searching the skies for rare and beautiful objects known as clusters of galaxies. It has become somewhat of an obsession for me and for an increasing number of researchers. The attraction of clusters is their potential to uncover some very fundamental things about the universe: its density, its geometry, its age and its ultimate destiny.
Clusters are made up of three major components: galaxies, hot gas and dark matter. The latter component provides most of the mass in the cluster roughly 80% and its nature remains one of the greatest mysteries in cosmology. It cannot be imaged directly, but its presence has been confirmed via a variety of techniques. The galactic component is illustrated in Figure 1, which shows a Hubble Space Telescope image of the cluster known as Abell 2218. From this image it is obvious how clusters got their name: they represent dense collections or clusterings of galaxies. Also obvious in the figure are white streaks that define incomplete ellipses around the cluster core. These streaks are distorted images of background galaxies. The powerful gravitation potential of the cluster is able to bend light much like an optical lens. The gaseous component is illustrated in Figure 2. Since the gas is very hot (about 108 Kelvin), it can be seen most easily in an X-ray image. Figure 2 is an image taken of Abell 2218 using the ROSAT X-ray satellite. Unlike the optical image, the X-ray image is rather featureless. This is a reflection of the fact that the gas is smoothly distributed throughout the cluster. This gas is known as the intracluster medium.
Technological advances are continuously improving our knowledge of clusters. One of the most significant advances in recent years has been the development of sensitive instruments that work in the millimeter (or microwave) part of the spectrum. This has allowed us to observe the "shadow" that clusters cast on the Cosmic Microwave Background Radiation. The blackbody spectrum of that radiation is altered slightly by interactions with the intracluster medium in a process known as the Sunyaev-Zel'dovich effect.
To try to explain how observations of clusters can be used to help us better understand the universe, here is a very simple description of how we think clusters form. The early universe was very dense and very smooth (to better than one part in one hundred thousand at the time the Cosmic Microwave Background Radiation was formed) but small density fluctuations did exist. These fluctuations were able to grow with time via gravitational attraction, by drawing in matter from the surrounding area. Most of this matter is thought to be the mysterious, invisible, dark-matter described above, but some of it would have been in the form of gas that was four parts hydrogen to one part helium. This gas provides the raw material for the visible components of the cluster: the intracluster medium and the galaxies. As the universe expands, the density of matter surrounding a cluster gets lower and lower and one can envisage a time at which a cluster has to stop growing because it is being starved of fresh material. Cut off from the rest of the universe, the galaxies will redden and fade as their stars burn off the last of their nuclear fuel. Correspondingly, the intracluster medium will slowly cool and its X-ray emission will become dimmer and dimmer.
By observing clusters we hope to be able to predict when (and if) this cold, dark, fate will befall us. A key component in this prediction is the measurement of the average density of the universe: the lower the density, the sooner the clusters will stop accreting fresh material. In other words, if we live in a low density universe, clusters must have reached their maximum mass a long time (maybe several billion years) ago. Alternatively, if our universe is very dense, clusters will still be accreting matter today and have yet to reach their maximum mass. In the high density case, one expects to see a rapid decrease in the number of clusters of any particular mass as we look further back in time. This rate of decrease has been calibrated using theoretical models and turns out to be very sensitive to the exact value of the density. Therefore, all one has to do to measure the density of the universe is to count the number of clusters as a function of mass and age (or distance).
Here at Carnegie Mellon, several projects are under way that seek to develop the "ultimate" cluster catalog, the one that will yield a definitive value for the density of the universe. My own efforts have concentrated on sifting through hundreds of ROSAT observations in search of very distant X-ray clusters. Bob Nichol is another veteran cluster hunter and has been recently developing sophisticated computer algorithms that will highlight clusters hidden in large galaxy catalogs. He hopes eventually to apply these algorithms to the largest galaxy catalog of them all, the Sloan Digitized Sky Survey, when it becomes available in a few years. Equally exciting are Jeff Peterson's plans to install a bolometer array on Carnegie Mellon's Viper Telescope (described in last year's Interactions), which will be able to detect extremely distant clusters via their Sunyaev Zel'dovich signal. Through this multi-wavelength approach (X-ray, optical and microwave) we hope that Carnegie Mellon will become a center of excellence for cluster research and will provide answers to some of the most fundamental questions that challenge cosmology today.
Figure 1. An optical image of cluster Abell 2218 taken with the Wide Field Camera on the Hubble Space Telescope. The image is 0.8x0.8 arcminutes in size. The curved white streaks in the figure are gravitationally lensed images of background galaxies. (Reproduced courtesy of W. Couch, R. Ellis and NASA.)
Figure 2. An X-ray image of cluster Abell 2218 taken with an X-ray camera on the ROSAT telescope. This image is 8x8 arcminutes in size. The bright central region coincides with the region shown in Figure 1. (Reproduced here courtesy of the SHARC collaboration.)
