The Stanford Cluster Search (StaCS) If you are a non-astronomer, you'd probably like to know what "clusters of galaxies" are. A famous example is shown in the picture below: the Coma Cluster, a "nearby" (a mere 300 million or so light years away) rich cluster in which hundreds of galaxies are found in a comparatively small volume of space. Galaxy clusters are unusual objects: their galaxies typically make up less than 5% of the total mass in the system. Ten to twenty percent of the total mass in in the form of an optically thin, very hot (more than ten million degrees Kelvin) plasma that fills the space in between the galaxies and emits vast quantities of X-rays. The X-ray emission is observable with specially designed satellite telescopes such as Chandra and XMM. The remainder of a cluster's mass is in the form of dark matter; the total mass in gas and galaxies doesn't provide enough gravity to hold the system together, so something else that we can't see must also be in the cluster. The StaCS project aims at finding clusters like Coma, but at distances up to fifty times larger-so distant, in fact, that the light just now reaching us was emitted when the universe was only half its present age. The very first distant cluster we identified is known as STACS J0104.8+0430, which is about twenty times more distant than the Coma Cluster.

• Investigators at Stanford University

  • Prof. Jeffrey A. Willick (deceased)
  • Dr. Keith L. Thompson

• Investigator at Livermore:

  • Dr. Benjamin Mathiesen

• Other Collaborators

  • Dr. Gary J. Hill, McDonald Observatory
  • Dr. Saul Perlmutter, Lawrence Berkeley National Laboratory
  • Dr. Robert A. Knop, Lawrence Berkeley National Laboratory

StaCS is a program to identify intermediate redshift (0.3 < z < 1) rich clusters of galaxies from deep, wide-field CCD images, and to follow-up these identifications with extensive spectroscopy at the Hobby-Eberly Telescope (HET). The clusters are identified using a matched-filter algorithm applied to images acquired as part of the Lawrence Berkeley Laboratory Supernova Cosmology Project (SCP). The SCP imaging data base covers approximately 25 square degrees of sky to a depth R = 23.5 or fainter.

Follow-up observations from the 8 meter class HET will measure cluster redshifts and velocity dispersion estimates, confirming the presence of real galaxy concentration. StaCS will pursue mass estimates via intensive follow-up spectroscopy at the HET and other telescopes, which we will seek to confirm with X-ray, Sunyaev-Zel'dovich, and gravitational lensing measurements. Our long-term goal is to determine the comoving number density of clusters as a function of mass and redshift, n(M,z). Comparison with theoretical predictions, using the Press-Schechter formalism and related approaches, will allow a determination of the mass density parameter _M, and the rms density fluctuation on an 8 Mpc scale, ₈, to 15% accuracy or better. These data will also be able to test the hypothesis that the primordial density fluctuations are Gaussian. See Willick 2000, ApJ 530, 80-95, for a discussion of how clusters can constrain nongaussianity.

In the short term, StaCS hopes to contribute a measurement of of cluster scaling relations at high redshifts by assembling a smaller sample of clusters and pursuing follow-up observations at Chandra and XMM. The global properties of the intracluster medium have been shown at low redshifts to fall along a tight relationship with very little scatter; the correlations between isophotal size, spectral temperature, and intracluster medium mass, for example, have a scatter of less than 20% about the best-fitting power law relations. The slopes of these scaling relations, however, do not agree with theoretical predictions. It seems that there are other mechanisms at work in the intracluster medium which alter the average thermodynamic properties of the gas, for example energy input from supernovae in the member galaxies or radiative cooling in the cluster cores. Most of the additional processes proposed to bring theoretical models of the scaling relations into line with observations also predict evolution in their slopes and normalizations, especially in the low-mass regime. Optical surveys of galaxy clusters have a much easier time finding low-mass systems at great distances than other methods, so StaCS has an important role to play in improving our understanding of the intracluster medium.

StaCS Science

An image of the first confirmed cluster, STACS J0104.8+0430, is shown here. The galaxies with known redshifts are marked, the cluster members shown in red and the non-cluster members in blue. The deep images of this field were from the Supernova Cosmology Project data base at LBL.

The spectra of these galaxies are shown here, and a histogram of the redshifts is shown here. These data were taken with the Hobby-Eberly Telescope in Ft. Davis, Texas using the Marcario Low-Resolution Spectrograph.

Here is a table of confirmed clusters which we will keep updated as new clusters are identified. Our candidate list right now includes about seventy clusters; the ones shown here all have at least two coincident redshifts.

StaCS Presentations and Publications

The first StaCS paper has been submitted to PASP and astro-ph, and can be downloaded in its current form.

Keith Thompson and Ben Mathiesen attended the Paris IAP conference, "Constructing the Universe with Galaxy Clusters" (July 2000), and presented work related to StaCS and the problem of measuring the cluster mass function. Keith's poster presenting an update of the program will appear soon. Here are the transparencies and conference proceedings for Ben's talk at this conference.

Keith Thompson and Ben Mathiesen attended the January 2000 meeting of the American Astronomical Society in Atlanta. Here are our two posters from this conference.

Our first cluster detection, z=0.401.

This work is supported in part by the Research Corporation.