MEASUREMENTS OF THE SUNYAEV-ZELDOVICH EFFECT IN THE NEARBY CLUSTERS A478, A2142, AND A2256

An X-ray flux-limited sample of nearby clusters of galaxies has been d efined for observations of the Sunyaev-Zeldovich effect (SZE) to be ca rried out on the Owens Valley 5.5 m telescope at 32 GHz. The X-ray sam ple selection minimizes the systematic errors introduced by cluster el ongation in the determination of H-0. Owing to their proximity, these clusters are well studied in the X-ray wave bands. The measurement of the SZE in three of these clusters is reported in this paper: Delta Ta u = -375 +/- 28 mu K (A478), -437 +/- 25 mu K (A2142), and -243 +/- 29 mu K (A2256). These values have been corrected for radio source conta mination, but have not been corrected for the beam dilution and switch ing (which are model dependent). There is an additional overall calibr ation uncertainty of 7%. If the temperature profile of the clusters is known, then the SZE provides a direct probe of the baryonic mass in t he hot ionized phase of the medium. We find surface baryonic mass dens ities of (7.5 +/- 2.5) x 10(13) M-. Mpc(-2) within the 7'.35 FWHM Gaus sian beam of the 5.5 m telescope projected on the cluster centers. For A2142, A2256, and the Coma cluster previously observed by Herbig et a l., we find a consistent value for the ratio of the SZE determined bar yonic mass to the gravitational binding mass of M-sze/M-tot = 0.061 +/ - 0.011 h(-1). Note that this is a lower limit on the total baryon fra ction, as there may be significant contributions from other baryons. C omparison with the standard big bang nucleosynthesis prediction Omega( B)h(2) = 0.013 +/- 0.02 gives a value for the cosmological density par ameter of Omega(0) h less than or similar to 0.21 +/- 0.05 assuming ou r limit on the baryon fraction in clusters applies to the universe as a whole. This density is in agreement with independently determined va lues from large-scale structure studies. Recent values for Omega(B) h( 2) based upon deuterium abundances are outside the previously accepted range, and in combination with our data lead to significantly higher or lower Omega(0). Finally, we present preliminary determinations of t he Hubble constant using X-ray models gleaned from the literature. The data from the three clusters, along with the results previously obtai ned using the same telescope on the Coma Cluster, yield a sample avera ge value H-0 = 54 +/- 14 km s(-1) Mpc(-1). We discuss the uncertaintie s in these results and future prospects for this method.