Measurements of the Sunyaev‐Zeldovich Effect in the Nearby Clusters A478, A2142, and A2256

Abstract

An X-ray flux-limited sample of nearby clusters of galaxies has been defined for observations of the Sunyaev-Zeldovich effect (SZE) to be carried out on the Owens Valley 5.5 m telescope at 32 GHz. The X-ray sample selection minimizes the systematic errors introduced by cluster elongation in the determination of H0. 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: ΔT = -375 ± 28 μK (A478), -437 ± 25 μK (A2142), and -243 ± 29 μK (A2256). These values have been corrected for radio source contamination, but have not been corrected for the beam dilution and switching (which are model dependent). There is an additional overall calibration uncertainty of 7%. If the temperature profile of the clusters is known, then the SZE provides a direct probe of the baryonic mass in the hot ionized phase of the medium. We find surface baryonic mass densities of (7.5 ± 2.5) × 1013 M☉ Mpc-2 within the 735 FWHM Gaussian beam of the 5.5 m telescope projected on the cluster centers. For A2142, A2256, and the Coma cluster previously observed by Herbig et al., we find a consistent value for the ratio of the SZE determined baryonic mass to the gravitational binding mass of Msze/Mtot = 0.061 ± 0.011 h-1. Note that this is a lower limit on the total baryon fraction, as there may be significant contributions from other baryons. Comparison with the standard big bang nucleosynthesis prediction ΩBh2 = 0.013±0.02 gives a value for the cosmological density parameter of Ω0h 0.21±0.05, assuming our limit on the baryon fraction in clusters applies to the universe as a whole. This density is in agreement with independently determined values from large-scale structure studies. Recent values for ΩBh2 based upon deuterium abundances are outside the previously accepted range, and in combination with our data lead to significantly higher or lower Ω0. Finally, we present preliminary determinations of the Hubble constant using X-ray models gleaned from the literature. The data from the three clusters, along with the results previously obtained using the same telescope on the Coma Cluster, yield a sample average value H0 = 54 ± 14 km s-1 Mpc-1. We discuss the uncertainties in these results and future prospects for this method.