Galaxy Clusters represent probably the largest structures in the Universe where gravity has won, where the density of galaxies and other matter is so large that the expansion of the Universe is stopped and a bound and possibly relaxed self-gravitating system has been formed. These objects, discovered early in the 20th century, soon after the discovery that individual galaxies represent objects well outside the Milky Way, serve both as laboratories for the study of galaxy-galaxy and galaxy-gas interactions and the determination of the mean mass density of the Universe and also as tracers of much larger structures.
Recent advances in astronomical detectors and techniques for classical groundbased telescopes as well as over two decades of space astronomy have produced an explosion of new information on galaxy clusters. The purpose of this website is to organize some of that data to enable astronmers, students, and other interested folks to see what clusters look like based primarily on redshifts but including other data. The basic data available are redshifts from the CfA Redshift Catalogue. Most of the raw data can be accessed via the ADC or anonymous ftp to fang.harvard.edu in /pub/catalogs (maintained by J. Huchra and J. Mader).
Our initial cluster sample was selected from the Abell Cluster Catalogue (Abell 1957) with the addition of the Virgo cluster. We chose to examine a semi-volume limited set of clusters selected by the magnitude of the 10th brightest cluster galaxy (empirically shown to be a good estimator of the distance of the cluster, e.g. Postman et al. 1985 and 1986.
Each of the clusters was examined through a Digitized Sky Survey image, a velocity histogram and a spatial plot of galaxies found within a projected 4.5 Mpc radius from Abell's coordinates. Images of our cluster sample may be found through this link.
A well-relaxed cluster should appear, on a velocity histogram, as a
peak with a Gaussian-like distribution. From velocity histograms and
spatial plots of the clusters, we subjectively determined an
appropriate search radius and range of velocities for each cluster.
The chosen cluster members were used to calculate dynamical
information, such as mean velocity and velocity dispersion, following
the example of Danese
Mass estimates were made following the example of Heisler, Tremaine, and Bahcall (1985). Five quantities were calculated: a harmonic radius, a virial mass, a projected radius, a projected mass using Abell center coordinates, and a projected mass using the calculated centroid. The results of these calculations, as well as some X-ray temperature data can be found through this link.
Abell, G. O. 1958, ApJS 3, 211.
Danese, L., De Zotti, G. and di Tullio, G. 1980, A&A, 82: 322.
Heisler, J., Tremaine, S., and Bahcall, J. 1985, ApJ, 298:8-17.
Postman, M., Geller, M., Huchra, J. and Henry, J. P 1990, ApJ.
Postman, et al. 1985, AJ 90, 1400.
Postman, et al. 1986, AJ 91, 1267.
John Huchra, Jacqueline Chen, Brian McNamara and Jeff Mader