K.S. Dawson, W.L. Holzapfel, E.D. Reese University of California at Berkeley, Berkeley, CA J.E. Carlstrom, S.J. LaRoque, D. Nagai University of Chicago, Chicago, IL M. Joy Marshall Space Flight Center, Huntsville, AL Formation of large scale structure in the universe is very sensitive to cosmological parameters such as baryon density, matter density, Hubble expansion, and the equation of state for dark energy. Galaxy clusters are the largest gravitationally bound structure in the universe and are considered an excellent target for observing the formation of large scale structure. To date, all galaxy clusters have been discovered in optical observations or X-Ray observations. However, both methods of observation are systematically biased against finding high redshift clusters. A third method of observing galaxy clusters, the Sunyaev-Zel’Dovich effect, was postulated soon after the discovery of the Cosmic Microwave Background (CMB). The (SZ) effect is a distortion of the CMB spectrum in the direction of a cluster of galaxies. The effect is independent of redshift and can be used to measure the size and mass of a cluster of galaxies. The BIMA array in Hat Creek, CA has been adapted to observe galaxy clusters through the SZ effect. The results from the BIMA array are the most sensitive SZ survey to date. SZ Effect in Clusters of Galaxies SZ Effect in Randomly Selected Fields The SZ Effect as a Probe for Galaxy Clusters Inverse scattering of CMB Photons by Hot Intracluster Plasma I cmb I cmb +  I SZ z~1100 t=300,000 years Two Components of the Electron Velocities Thermal (Te~100,000,000 K) Bulk Motion (Doppler Shift) Produce Two Components of the SZ effect  of CMB is scattered e-e-  Cluster Springel, White,Hernquist2001 HydrodynamicalSimulation of 1 square degree of SZ sky Free electrons and ionized nuclei fall into the gravitational well of a cluster of galaxies. The charged particles gain large amounts of energy (several keV) in the process. This energy is transferred from the hot intracluster plasma to the cold CMB photons though Compton scattering. The process is referred to as the Sunyaev-Zel’Dovich (SZ) effect. The thermal SZ effect is observed through distortions of the CMB spectrum in the direction of a galaxy cluster. Unlike X-ray and optical observations, the SZ effect is independent of redshift. This property should allow an SZ survey to identify galaxy clusters that are too distant to be discovered in an X-Ray survey, producing a more complete catalog of clusters. To date only the biggest and brightest clusters have been mapped through the SZ effect with the BIMA and OVRO arrays. These have been follow up observations to clusters discovered in X-Ray and optical surveys. Observing clusters over a wide range of redshift is useful in calculating the rate of expansion of the universe. Calculations of Hubble constant have been consistent with other methods. Data from follow-up observations can also be combined with X-Ray data to derive cluster masses and baryon density. Data is currently being analyzed to compare masses derived from SZ and X-Ray observation to masses derived from optical lensing. The Berkeley-Illinois-Maryland Association (BIMA) array is designed for mm-wave observations. During the summer, these telescopes are outfitted with sensitive cm-wave receivers. The resulting synthesized beam of the BIMA telescope is approximately 2 arcminutes using nearest neighbor telescopes. Sensitivity to arcminute scale structure is essential for imaging clusters of galaxies. Hydrodynamical simulations predict a rate of structure formation that has a strong dependence on cosmological parameters. Observations in the next few years are expected to reveal number counts in the thousands, creating a catalog of clusters with the potential to constrain cosmological models. Cluster surveys probe (1) volume-redshift relation, (2) abundance evolution, (3) structural evolution We have performed the most sensitive SZ survey to date with the BIMA telescope. The survey covers 0.1 square degrees. Data collected in the summer of 2002 doubles the sky coverage and is being analyzed now. One only expects several low signal to noise clusters to lie in the BIMA survey. It is therefore useful to determine a power spectrum from the raw data. Analysis of a power spectrum will include contributions from clusters that lie near the noise level. Ten 6.1 Meter Dishes 6.3’ Primary beam Close Pack 2D array 28.5GHz HEMT Receivers T sys (summer) ~40K 800 MHz Digital Correlator Only the CBI experiment run by Cal Tech and the BIMA experiment measure the power spectrum on scales at which the SZ effect is expected to dominate. The addition of this year’s data should lower the uncertainty in the BIMA measurements by a factor of two. This improvement may be enough to constrain certain cosmological models. Radio point sources, such as active galactic nuclei (AGN), contaminate CMB observations by contributing to excess power. To account for point source contamination, each BIMA field is observed at 5 GHz with the VLA. Point sources are identified with the VLA observations and removed from the analysis of the BIMA data. Since point sources are expected to be much brighter at the low frequency, we expect to remove all point sources which lie near the noise levels of the BIMA images. It would not be possible to remove these point sources with the BIMA data alone. The window function describes the angular scales on which the experiment is sensitive. The BIMA data is divided into two bins, one centered at l=5500, corresponding to angular scales of 2 arcminutes, the other centered at l=9500, corresponding to an angular scale of 1 arcminute. The third curve represents the sum of the two bins. Window Functions for BIMA Analysis Likelihood Functions of BIMA Analysis The likelihood function describes the probability that the data is described by a given level of excess power. The function is normalized with respect to the scenario described by zero excess power. l=5500 l=6500 l=9500