The Stanford Cluster Search for Distant Galaxy Clusters

Jeffrey A. Willick (deceased), Keith L. Thompson, Benjamin F. Mathiesen, Saul Perlmutter, Robert A. Knop, and Gary J. Hill

We describe the scientific motivation behind, and the methodology of, the Stanford Cluster Search (StaCS), a program to compile a catalog of optically selected clusters of galaxies at intermediate and high (0.3 < z < 1) redshifts. The clusters are identified using a matched filter algorithm applied to deep CCD images covering

60 square degrees of sky. These images are obtained from several data archives, principally that of the Berkeley Supernova Cosmology Project of Perlmutter et al. Potential clusters are confirmed with spectroscopic observations at the 9.2 m Hobby-Eberly Telescope. Follow-up observations at optical, sub-mm, and X-ray wavelengths are planned in order to estimate cluster masses. Our long-term scientific goal is to measure the cluster number density as a function of mass and redshift, n(M,z), which is sensitive to the cosmological density parameter

_m and the amplitude of density fluctuations

₈. Our short term goals are the detection of high-redshift clusters over a broad mass range and the measurement of evolution in cluster scaling relations. The combined data set will contain clusters ranging over an order of magnitude in mass, and allow constraints on these parameters accurate to

10%. We present our first spectroscopically confirmed cluster candidates and describe how to access them electronically.

Comments: The long-term goal of measuring _m with the StaCS catalogue is probably now untenable, unless both Keith Thompson and Ben Mathiesen obtain positions with generous funding and freedom of research. Over the next year or two, however, we should be able to assemble a catalogue of two dozen confirmed StaCS clusters in the range 0.4 < z < 1.0. This is an important scientific goal for the following reasons:

The number of known clusters above z = 0.4 is still rather small. The number of known clusters above z=0.7 is very small, and many more objects need to be detected before we can even guess at what the cluster population is like at these redshifts.
Optical surveys such as StaCS have an advantage over X-ray and S-Z surveys in that they more easily detect poor clusters. Most of the physical processes that can break the self-similarity of cluster scaling relations have a stronger effect on poor clusters than rich clusters, and a sample of high-redshift poor clusters (M_tot about 2 x 10¹⁴ solar masses) is therefore necessary to properly model the cluster luminosity and temperature functions. Most of the clusters in the StaCS catalogue will be poor clusters.
The fact that our candidates are spectroscopically confirmed makes them ideal candidates for the X--ray or S-Z follow-up observations needed to understand their properties.

It is therefore our hope that even an incomplete StaCS catalogue will provide a valuable resource to the scientific community: a list of spectroscopically confirmed, high-redshift poor clusters which will be available to everyone. We intend to propose for HST, Chandra, and XMM time in order to eventually assemble a measurement of intracluster medium scaling relations for poor clusters, and all the rest of you are invited to do the same.

Willick et al., 2001, PASP, 113, 658. astro-ph/0012119

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Last modified: Wed Sep 19 17:24:23 PDT 2001